Dota2 benchmark regression

[kvark]

Somewhere between https://gfx-rs.github.io/2018/08/10/dota2-macos-performance.html and today we regressed the benchmark significantly. We used to get 33.9 / 3.5 (in the table), but now we are closer to just 30 (in immediate mode). Here are the things to try:

• [x] new swapchain model is less efficient at fullscreen? Tried with "-sw" parameter, no change.
• [ ] Dota doesn't discover some of the features we have, it needs to support VK_KHR_portability_subset
• [x] on macOS 11 we aren't using some of the Metal features we are supposed to use
• [x] we are locking too much, and that hurts on macOS11 beta, possibly because of parking_lot regression? Tried parking_lot-0.6, no change.
• [x] macOS 11 beta has some strange scheduling issues. Maybe background apps interfere too much? 🥇 disabling Firefox in the background gets us from 30 to 32fps in average.
• [x] setting labels on internal encoders is slow - hid them behind a flag now, not sure if changed any metrics
• [ ] ExactSizeIterator bounds are confusing the compiler and making us heap-allocate more?
• [ ] Rust optimization regression?
• [x] creating and filling MTLRenderPassDescriptor is too slow - implemented a pool for reusing them in https://github.com/gfx-rs/gfx/pull/3379/commits/bbf55ff085dd676eb6a214edacc92b6072776814
• [x] binding descriptor sets is still to slow - rewritten in https://github.com/gfx-rs/gfx/pull/3387/commits/15e456b1941c2e02213aa0097be7ed9458aa660e
• [x] too many redundant depth-stencil changes - fixed in https://github.com/gfx-rs/gfx/pull/3387/commits/0d581a6c33a4b8b6996c416e3f5865a01521ad8f
• [x] redundant scissor changes - fixed in https://github.com/gfx-rs/gfx/pull/3387/commits/f0051da612494931002b03fe5450f228a49dd87d
• [ ] we are doing many redundant passes - investigated, no clear picture of why that would happen
• [x] we are rebinding too many resources - testing a fix but it gave no performance difference. We are binding them in ranges, anyway, so Metal run-time possibly does this for us.
• [x] we are using mutable pixel views too often - fixed in https://github.com/gfx-rs/gfx/pull/3379/commits/22e2f4d73d9b74d22254e334d9880ea2d1616402

Related to https://github.com/gfx-rs/gfx/issues/3378, https://github.com/gfx-rs/gfx/issues/3382, https://github.com/gfx-rs/gfx/issues/3381, https://github.com/gfx-rs/gfx/issues/3383

[kvark]

Dota doesn't discover some of the features we have

I just noticed that it does indeed print the following lines when ran on gfx-portability:

Vulkan physical device (0): does not support Metal depthSampleCompare. Vulkan physical device (0): using transform constant buffer: false Vulkan physical device (0): supports shader clip distance: true Vulkan physical device (0): using secondary command buffers: false

I imagine working around the comparison samplers has a solid GPU performance cost, and on Intel GPU it would show up as an overall regression. So I believe this issue is blocked on https://github.com/ValveSoftware/Dota-2-Vulkan/issues/351