[rlgl] Remove GLEW library dependency (if possible)

[raysan5]

In order to reduce raylib external dependencies, OpenGL extensions loading library GLEW could be replaced by a lightweight alternative to load only OpenGL 3.3 Core profile required extensions.

Related functions [rlgl.c]:

void rlglInit(void);

Some alternatives have been already tested: glad and glLoadGen. Those libraries generate custom headers to be included in the project with only the required functionality.

glad seems the most interesting alternative but it's not working (neither glLoadGen), #include <windows.h> must be removed due to conflicts with raylib, but glad compiles anyway... When running any example, it crashes.

[raysan5]

After further investigation and testing, I realized a couple of things.

Extensions initialization should be moved from rlgl.c to core.c function InitDisplay(), just below function call glfwMakeContextCurrent(window).

That way, all required GL functions will be loaded before InitGraphics().

I confirm omitting #include <windows.h> on glad.h breaks the program (despite compilation with no complaints).

[raysan5]

Issue solved on commit 4e57bd1

Thanks to @Dav1dde directions, GLEW was finally replaced by GLAD (Check Dav1dde/glad#42).

GLEW alternative has also been left in the code (temporaly) while checking some weird behaviours on gDEBugger/CodeXL profilers.