GPUExtensions: GL backend isolation

This is part of the Vulkan task T68990.

This commits changes a few things:
- Rename extensions to capabilities (but left the file name untouched).
- Cubemap mip render workaround detection is rewritten using gl
  commands to avoid using the GPU API before initialization.
- Put all the capabilities that are only relevant for the GL backend
  inside GLContext as static variables.
- Cleanup the names of the limit variables.
- Separate all GL related workaround search inside the GL module.

source/blender/gpu/intern/gpu_init_exit.c

@@ -48,7 +48,6 @@ void GPU_init(void)
   }
 
   initialized = true;
-  gpu_extensions_init(); /* must come first */
 
   gpu_codegen_init();
   gpu_material_library_init();
@@ -79,8 +78,6 @@ void GPU_exit(void)
   gpu_material_library_exit();
   gpu_codegen_exit();
 
-  gpu_extensions_exit();
-
   initialized = false;
 }