WIP Introduce WLR_GBM_EXPLICIT_UPGRADE

When the `WLR_GBM_EXPLICIT_UPGRADE` flag is set to 1, the GBM allocator
will attempt to "upgrade" buffers allocated with implicit modifiers to
be managed with explicit modifiers.

This is useful to enable the conservative modifier fallback for the
Vulkan renderer on drivers/hardware that fully support implicit and
explicit modifiers, including being able to report the modifier of an
implicitly allocated BO.

include/render/allocator/gbm.h

@@ -22,6 +22,10 @@ struct wlr_gbm_allocator {
 	struct gbm_device *gbm_device;
 
 	struct wl_list buffers; // wlr_gbm_buffer.link
+
+	struct {
+		bool explicit_upgrade;
+	} WLR_PRIVATE;
 };
 
 /**