wgpu_hal::Surface::discard_texture broken on Vulkan

[jimblandy]

The implementation of wgpu_hal::Surface::discard_texture in the vulkan backend does nothing. If it is ever called, presumably some subsequent call to acquire_texture will hang, since the texture will never be re-used.

Since wgpu_hal only permits the acquisition of a single surface texture at a time, it should be sufficient for the discard_texture implementation to simply save the texture away somewhere and hand it out again from the next acquire_texture call.

This issue was noticed in the review for #5681.