Add conditional shader compilation

[tomas7770]

Problem

Some shaders have parts of them that would be useful to configure from the engine, but can't be configured without changing the shader code and recompiling, e.g. number of CSM splits in deferred shading, textures drawn to in the G-buffer rasterizer (to disable/enable the render picker).

Solution

Features could be controlled using #define macros prepended to the shader code, e.g. having #define RENDER_PICKER, and then checking with #ifdef RENDER_PICKER. The Shader asset would store the source code, provide functions to set the values, e.g. assets.read(VertexShader)->builder().with("RENDER_PICKER").with("MAX_CSM_SPLITS", "5").build();, and then recompile the modified code.