render/vulkan: allow rendering to RGB565 buffers

This is implemented by a two-subpass rendering scheme; the first
subpass draws (and blends) onto a linear R16G16B16A16_SFLOAT buffer,
while the second subpass performs linear->srgb conversion, writing
onto the actual output buffer.
3 jobs for vk-output-5 in 1 minute and 56 seconds (queued for 2 seconds)