Metal: Always add GPU_TEXTURE_USAGE_ATTACHMENT flag

Texture clearing is done using framebuffer clear.
To avoid adding the flag everywhere we add it as part
of the texture creation process.

This should have no performance impact.

source/blender/gpu/intern/gpu_texture.cc

@@ -178,6 +178,9 @@ bool Texture::init_view(GPUTexture *src_,
 void Texture::usage_set(eGPUTextureUsage usage_flags)
 {
   gpu_image_usage_flags_ = usage_flags;
+  /* Metal: Texture clearing is done using framebuffer clear. This has no performance impact. */
+  /* TODO(fclem): Move this to metal backend instead to avoid side effects in other backends. */
+  gpu_image_usage_flags_ |= GPU_TEXTURE_USAGE_ATTACHMENT;
 }
 
 /** \} */