Update gfx with the image-less framebuffer semantics

[kvark]

Contains the following major pieces in the gfx-hal update:

• use gfx's new image-less framebuffers - https://github.com/gfx-rs/gfx/pull/3571. Note: we aren't actually exposing VK_imageless_framebuffer yet, but this will be in follow-ups.
• update the external synchronization semantics from gfx - https://github.com/gfx-rs/gfx/pull/3553 . Note: the main benefit is extracted at gfx-rs level.
• the dependencies for smallvec and parking_lot are dropped (yay!)

[kvark]

The swapchain code is much simpler, and we have 2 less dependencies. Looks like a clear win! I tested Dota, seems to work as usual 😎 bors r+

[bors[bot]]

Build succeeded:

• iOS Stable
• MacOS Stable
• Ubuntu Nightly
• Windows Stable