GPUPlatform: GL backend isolation

Part of the vulkan implementation T68990.

Pretty straight forward. Just move the GL code inside the GLBackend and
make the GPUPlatformGlobal a class object.

source/blender/gpu/intern/gpu_init_exit.c

@@ -48,7 +48,6 @@ void GPU_init(void)
   }
 
   initialized = true;
-  gpu_platform_init();
   gpu_extensions_init(); /* must come first */
 
   gpu_codegen_init();
@@ -81,7 +80,6 @@ void GPU_exit(void)
   gpu_codegen_exit();
 
   gpu_extensions_exit();
-  gpu_platform_exit(); /* must come last */
 
   initialized = false;
 }