Add support for extension VK_EXT_headless_surface.

[billhollings]

• Consolidate info about CAMetalLayer and headless in MVKSurface.
• MVKSwapchainImage remove getCAMetalDrawable() and focus on abstracting getMTLTexture().
• MVKPresentableSwapchainImage::getCAMetalDrawable() return nil if headless.
• Add MVKPresentableSwapchainImage::_mtlTextureHeadless to support a fixed MTLTexture that is not retrieved from a CAMetalDrawable.
• MVKPresentableSwapchainImage refactor signalling semaphores and fences.
• MVKPresentableSwapchainImage don't lock when signalling semaphores and fences.
• If no present occurs, actualPresentTime will be zero. Set it to current time, instead of to desiredPresentTime, since it's more accurate.
• Rework timestamps:
  • Remove _mvkTimestampBase so mvkGetTimestamp() is equal to mach_absolute_time(), which is used in presentation timing.
  • Add mvkGetRuntimeNanoseconds().
  • Rename mvkGetAbsoluteTime() to mvkGetContinuousNanoseconds().
  • Remove mvkGetTimestampPeriod() as unused.
• MVKSemaphoreMTLEvent::encodeDeferredSignal remove redundant nil test (unrelated).
• Fix swapchain and surface bugs when windowing system is accessed from off the main thread (unrelated).
• Log warning when deprecated functions vkCreateMacOSSurfaceMVK()
  or vkCreateIOSSurfaceMVK() are used (unrelated).
• Remove documentation for visionos, as support is not ready (unrelated).

Fixes #2049.
Fixes #2085.

Passes all related CTS WSI tests.

[billhollings]

@cdavis5e

The (now renamed) mvkGetContinuousNanoseconds() was introduced for VK_EXT_calibrated_timestamps. Is there a reason why that time must be continuous (mach_continuous_time())? Or can it be replaced with mvkGetRuntimeNanoseconds()?

[cdavis5e]

Is there a reason why that time must be continuous (mach_continuous_time())?

Yes. This is the function that's used for the CLOCK_MONOTONIC_RAW domain. VK_EXT_calibrated_timestamps doesn't have CLOCK_UPTIME or CLOCK_UPTIME_RAW domains. See the comment starting on line 1182 of MVKDevice.mm.

[cdavis5e]

A minor grammatical error and... well, a preëxisting bug, but since you're changing that code...

[billhollings]

Thanks for noticing the connection to #2085.

While I was in there, I made a couple of additional changes about windowing systems. And while I was doing that, I also removed the documentation for visionos support, as it's really not properly supported and doesn't build correctly. Also added a deprecation warning for the old macOS and iOS surface creation extensions.