Keep track of the last bound descriptor set (Vulkan backend improvement)

[micb25]

This PR introduces an improvement in the Vulkan backend which reduces descriptor set changes. A local variable (last_desc_set) tracks the last bound descriptor set and avoids a rebinding the identical descriptor set. For my application, this reduced the vkCmdBindDescriptorSets calls by 67% from 19 to 7.

[ocornut]

Thank you for your PR.

this reduced the vkCmdBindDescriptorSets calls by 67% from 19 to 7.

Were you able to meaningful measure the cost of those 19 or 7 calls in a profiler?

[micb25]

Thanks for your question. For these benchmarks I used a bit more complex scene that ended up in 7 vs 23 calls (patched vs unpatched) to vkCmdBindDescriptorSets. On the CPU side, it took in average 5.7 µs vs 6.4 µs (-11%) to call ImGui::Render(). On the GPU side, however, profiling with NSight Graphics didn't measure any differences (both 0.19 ms for UI rendering on a NVIDIA RTX 2060). I assume that the GPU driver keeps track of descriptor changes itself (at least NVIDIA).

Edit: I accidentally added two more commits to this PR. They have been reverted.

[ocornut]

For these benchmarks I used a bit more complex scene that ended up in 7 vs 23 calls (patched vs unpatched) to vkCmdBindDescriptorSets. On the CPU side, it took in average 5.7 µs vs 6.4 µs (-11%) to call ImGui::Render().

Honestly this sounds not meaningful and barely measurable, but I've narrowed down the patch as 4 lines and it doesn't seem to hurt to do it for now. Merged as 90025a6.