layers: Further rework to AllocateDescriptorSets
layers: Further rework to AllocateDescriptorSets

- operator new does not return null, it throws.
- do lookups once, at the start

Signed-off-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>

Radamanthe/VulkanSamples,Radamanthe/VulkanSamples,Radamanthe/VulkanSamples,Radamanthe/VulkanSamples,Radamanthe/VulkanSamples,Radamanthe/VulkanSamples