vulkan: Don't try that hard to use clear

vulkan: Don't try that hard to use clear

For small regions, the optimization doesn't matter that much, so we
don't need to do lots of work on the CPU.

In particular, this should catch icons and their backgrounds (32x32),
but I was generous in selecting the number.

Gets my discrete AMD on widget-factory back to the 1900fps it had before
this optimization while making the driver clock the GPU's shader at
1.7GHz instead of the 2.1GHz it used before.