Use OpenGL GLVND ABI instead of legacy ABI

This commit forces the project to use the vendor-neutral OpenGL loading
library as well as the windowing system agnostic interface to OpenGL.
It should be supported by all target platforms.

CMake/Backends.cmake

@@ -21,7 +21,7 @@ target_sources(rmlui_backend_Win32_GL2 INTERFACE
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Renderer_GL2.h"
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Include_Windows.h"
 )
-target_link_libraries(rmlui_backend_Win32_GL2 INTERFACE rmlui_backend_common_headers OpenGL::GL)
+target_link_libraries(rmlui_backend_Win32_GL2 INTERFACE rmlui_backend_common_headers OpenGL::OpenGL OpenGL::EGL)
 
 add_library(rmlui_backend_Win32_VK INTERFACE)
 target_sources(rmlui_backend_Win32_VK INTERFACE    
@@ -50,7 +50,7 @@ target_sources(rmlui_backend_X11_GL2 INTERFACE
 # be substituted by "X11:X11" in addition to any of the other imported that might
 # be required. More info: 
 # https://cmake.org/cmake/help/latest/module/FindX11.html
-target_link_libraries(rmlui_backend_X11_GL2 INTERFACE rmlui_backend_common_headers OpenGL::GL ${X11_LIBRARIES})
+target_link_libraries(rmlui_backend_X11_GL2 INTERFACE rmlui_backend_common_headers OpenGL::OpenGL OpenGL::EGL ${X11_LIBRARIES})
 
 add_library(rmlui_backend_SDL_GL2 INTERFACE)
 target_sources(rmlui_backend_SDL_GL2 INTERFACE    
@@ -60,7 +60,7 @@ target_sources(rmlui_backend_SDL_GL2 INTERFACE
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Platform_SDL.h"
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Renderer_GL2.h"
 )
-target_link_libraries(rmlui_backend_SDL_GL2 INTERFACE rmlui_backend_common_headers OpenGL::GL SDL2::SDL2 GLEW::GLEW SDL2_image::SDL2_image)
+target_link_libraries(rmlui_backend_SDL_GL2 INTERFACE rmlui_backend_common_headers OpenGL::OpenGL OpenGL::EGL SDL2::SDL2 GLEW::GLEW SDL2_image::SDL2_image)
 
 add_library(rmlui_backend_SDL_GL3 INTERFACE)
 target_sources(rmlui_backend_SDL_GL3 INTERFACE    
@@ -71,7 +71,7 @@ target_sources(rmlui_backend_SDL_GL3 INTERFACE
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Renderer_GL3.h"
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Include_GL3.h"
 )
-target_link_libraries(rmlui_backend_SDL_GL3 INTERFACE rmlui_backend_common_headers OpenGL::GL SDL2::SDL2 SDL2_image::SDL2_image)
+target_link_libraries(rmlui_backend_SDL_GL3 INTERFACE rmlui_backend_common_headers OpenGL::OpenGL OpenGL::EGL SDL2::SDL2 SDL2_image::SDL2_image)
 if(UNIX)
     # The OpenGL 3 renderer implementation uses dlopen/dlclose
     # This is required in some UNIX and UNIX-like operating systems to load shared object files at runtime
@@ -114,7 +114,7 @@ target_sources(rmlui_backend_SFML_GL2 INTERFACE
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Renderer_GL2.h"
 )
 target_link_libraries(rmlui_backend_SFML_GL2 INTERFACE 
-    rmlui_backend_common_headers OpenGL::GL rmlui_SFML_old_wrapper SFML::Graphics SFML::Window SFML::System
+    rmlui_backend_common_headers OpenGL::OpenGL OpenGL::EGL rmlui_SFML_old_wrapper SFML::Graphics SFML::Window SFML::System
 )
 if(WIN32)
     target_link_libraries(rmlui_backend_SFML_GL2 INTERFACE SFML::Main)
@@ -128,7 +128,7 @@ target_sources(rmlui_backend_GLFW_GL2 INTERFACE
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Platform_GLFW.h"
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Renderer_GL2.h"
 )
-target_link_libraries(rmlui_backend_GLFW_GL2 INTERFACE rmlui_backend_common_headers OpenGL::GL glfw)
+target_link_libraries(rmlui_backend_GLFW_GL2 INTERFACE rmlui_backend_common_headers OpenGL::OpenGL OpenGL::EGL glfw)
 
 add_library(rmlui_backend_GLFW_GL3 INTERFACE)
 target_sources(rmlui_backend_GLFW_GL3 INTERFACE    
@@ -139,7 +139,7 @@ target_sources(rmlui_backend_GLFW_GL3 INTERFACE
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Renderer_GL3.h"
     "${PROJECT_SOURCE_DIR}/Backends/RmlUi_Include_GL3.h"
 )
-target_link_libraries(rmlui_backend_GLFW_GL3 INTERFACE rmlui_backend_common_headers OpenGL::GL glfw)
+target_link_libraries(rmlui_backend_GLFW_GL3 INTERFACE rmlui_backend_common_headers OpenGL::OpenGL OpenGL::EGL glfw)
 if(UNIX)
     # The OpenGL 3 renderer implementation uses dlopen/dlclose
     # This is required in some UNIX and UNIX-like operating systems to load shared object files at runtime

CMake/BackendsDependencies.cmake

@@ -197,16 +197,16 @@ endif()
 # OpenGL handling changes in CMake 3.11, requiring to set CMake policy CMP0072
 # More info: https://cmake.org/cmake/help/latest/policy/CMP0072.html
 if(RMLUI_SAMPLES_BACKEND MATCHES "GL2$")
-    find_package("OpenGL" "2")
+    find_package("OpenGL" "2" COMPONENTS "OpenGL" "EGL")
     if(NOT TARGET OpenGL::GL)
-        report_not_found_dependency("OpenGL" OpenGL::GL)
+        report_not_found_dependency("OpenGL" OpenGL::OpenGL)
     endif()
 endif()
 
 if(RMLUI_SAMPLES_BACKEND MATCHES "GL3$")
-    find_package("OpenGL" "3")
+    find_package("OpenGL" "3" COMPONENTS "OpenGL" "EGL")
     if(NOT TARGET OpenGL::GL)
-        report_not_found_dependency("OpenGL" OpenGL::GL)
+        report_not_found_dependency("OpenGL" OpenGL::OpenGL)
     endif()
 endif()