Split rlgl.h into multiple files with a common interface

[ValorZard]

Splitting rlgl.h would let you more easily add a backend for vulkan or sdl_gpu or whatever

related (kinda) to #2048

basically how I would think it would work is that for all the ifdefs in rlgl, you could put the implementations in a seperate file and then inline it maybe? Idk

[raysan5]

@ValorZard I'm afraid rlgl.h is currently very mapped to OpenGL, supporting other graphic APIs would require a big redesign of the module and also raylib. Out of scope for now.

[ValorZard]

Understandable