Move CPU/GPU timestamp correlation to vkGetPhysicalDeviceProperties()

[billhollings]

It turns out that MTLDevice sampleTimestamps:gpuTimestamp: executes synchronously with other queue activities, and can block GPU execution if it is called frequently.

Move timestamp correlation to the execution of vkGetPhysicalDeviceProperties() so it is called only on demand by the app.

Furthermore, MTLDevice sampleTimestamps:gpuTimestamp: should only be invoked if all of these conditions are met (otherwise the VkPhysicalDeviceLimits::timestampPeriod should be set to 1.0):

• CPU is not Apple Silicon (!supportsMTLGPUFamily(Apple1)).
• MVKPhysicalDevice::_timestampMTLCounterSet exists.
• MVKPhysicalDeviceMetalFeatures::counterSamplingPoints is empty.

[billhollings]

PR #1992 fixes this issue. Please retest to verify, and close this issue if it fixes the problem you encountered.