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/intern/gpu_framebuffer.cc

@@ -96,10 +96,7 @@ void FrameBuffer::attachment_set(GPUAttachmentType type, const GPUAttachment &ne
 
   if (new_attachment.tex) {
     if (new_attachment.layer > 0) {
-      BLI_assert(ELEM(GPU_texture_target(new_attachment.tex),
-                      GL_TEXTURE_2D_ARRAY,
-                      GL_TEXTURE_CUBE_MAP,
-                      GL_TEXTURE_CUBE_MAP_ARRAY_ARB));
+      BLI_assert(GPU_texture_cube(new_attachment.tex) || GPU_texture_array(new_attachment.tex));
     }
     if (GPU_texture_stencil(new_attachment.tex)) {
       BLI_assert(ELEM(type, GPU_FB_DEPTH_STENCIL_ATTACHMENT));