[graphite] Allocate more storage in Vulkan texture keys for image usage flags

* When toggling through images in Viewer and switching from GM_aarectmodes -> GM_aaxfermodes, we failed an assertion checking that VkImageUsageFlags fit within their allocated portion of the texture key

* We were not using the entire uint32 provided, so just let the image flags fill the rest of the space.

* The viewer result is still not perfect, but this fixes a fatal crash cause by a failed assertion.

Change-Id: Id9395e8128453bc6fc17640c75ecf6ef647951c9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/734341
Reviewed-by: Jim Van Verth <jvanverth@google.com>
Commit-Queue: Nicolette Prevost <nicolettep@google.com>
1 file changed