Defer resetting MTLTexture in MVKSwapchainImage until request to acqu…

…ire.

Move call to MVKSwapchainImage::resetMetalDrawable() from immediately after
presentation until request to acquire, to preserve image content for copying
after presentation in cases such as an app screen capture operation.

Testing indicates this deferral has little or no adverse effect on the performance
of retrieving the drawable when next needed.

MoltenVK/MoltenVK/GPUObjects/MVKImage.mm

@@ -958,7 +958,6 @@
 	[mtlCmdBuff presentDrawable: getCAMetalDrawable()];
 	if (scName) { mvkPopDebugGroup(mtlCmdBuff); }
 
-	resetMetalDrawable();
 	signalPresentationSemaphore(mtlCmdBuff);
 
 	retain();	// Ensure this image is not destroyed while awaiting MTLCommandBuffer completion

MoltenVK/MoltenVK/GPUObjects/MVKSwapchain.mm

@@ -97,6 +97,7 @@
 
 	// Return the index of the image with the shortest wait and signal the semaphore and fence when it's available
 	*pImageIndex = minWaitImage->_swapchainIndex;
+	minWaitImage->resetMetalDrawable();
 	minWaitImage->signalWhenAvailable((MVKSemaphore*)semaphore, (MVKFence*)fence);
 
 	return getHasSurfaceSizeChanged() ? VK_ERROR_OUT_OF_DATE_KHR : VK_SUCCESS;