Fix image usage for screenshot

[yknishidate]

Description

I fixed an issue where taking screenshots caused a validation error because the swapchain image usage did not include TRANSFER_SRC.

[error] -8823274 - VUID-VkImageMemoryBarrier-oldLayout-01212: vkCmdPipelineBarrier(): pImageMemoryBarriers[0].newLayout (VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL) is not compatible with VkImage 0x380000000038 usage flags VK_IMAGE_USAGE_TRANSFER_DST_BIT|VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT.

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[SaschaWillems]

Afaik blit source is not mandatory for (some of) the formats we use for the swapchain. So this should only add said flag if that format flag is supported.

[yknishidate]

Thanks for the comment!

I agree that TRANSFER_SRC should not be used if it is not supported by the surface.

However, from what I understand, swapchain images include TRANSFER_SRC by default in the common swapchain setup:

Vulkan-Samples/framework/core/swapchain.h

Line 97 in 51c0943

const std::set<VkImageUsageFlagBits> &image_usage_flags = {VK_IMAGE_USAGE_COLOR_ATTACHMENT_BIT, VK_IMAGE_USAGE_TRANSFER_SRC_BIT},

The validation error is caused by update_swapchain_image_usage_flags() dropping TRANSFER_SRC from the default usage. My intention was to preserve the flag required by the screenshot, not to blindly add an unsupported usage flag.

Also, the support check is already performed when recreating the swapchain, so I think it should be reasonable for the sample code to request TRANSFER_SRC without performing its own capability check, similar to the setup process.

NOTE: As a possible follow-up improvement, it might be worth considering the requested image usage flags during swapchain format selection, instead of selecting the format first and validating the usage afterward.