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/opengl/gl_backend.hh

@@ -50,6 +50,7 @@ class GLBackend : public GPUBackend {
     /* platform_init needs to go first. */
     GLBackend::platform_init();
 
+    GLBackend::capabilities_init();
     GLTexture::samplers_init();
   }
   ~GLBackend()
@@ -127,6 +128,8 @@ class GLBackend : public GPUBackend {
  private:
   static void platform_init(void);
   static void platform_exit(void);
+
+  static void capabilities_init(void);
 };
 
 }  // namespace gpu