GPUFramebuffer: Encapsulate single attachement clear

This is in preparation of using it to clear single texture.

Also includes minor cleanups about not using tex target in
assert and adding enum operators.

source/blender/gpu/opengl/gl_framebuffer.hh

@@ -81,6 +81,9 @@ class GLFrameBuffer : public FrameBuffer {
              float clear_depth,
              uint clear_stencil) override;
   void clear_multi(const float (*clear_cols)[4]) override;
+  void clear_attachment(GPUAttachmentType type,
+                        eGPUDataFormat data_format,
+                        const void *clear_value) override;
 
   void read(eGPUFrameBufferBits planes,
             eGPUDataFormat format,