Avoid linking libGL at compile time

[katyo]

I see that OpenGL library linking with nanovg-sys at compile time:

nanovg-rs/nanovg-sys/build.rs

Lines 20 to 23 in 5d85f76

	if target.contains("linux") {
	println!("cargo:rustc-link-lib=GL");
	} else if target.contains("darwin") {
	println!("cargo:rustc-link-lib=framework=OpenGL");

Why we need to do it so?
I think we may and should avoid to do it because usually OpenGL API procedures initializes dynamically at runtime by requesting addresses to it after initializing context.

[katyo]

It seems I found reasonable solution.
I suggest simply use GLAD for GL API calls.
The GLAD allows load addresses of GL procedures using external function, which can be provided from rust-side.
I hope that solution may help resolve my previous issue #80.
Are you welcome PRs?

[KevinKelley]

PR welcome, yes..