Using renderpass layout transitions is more optimal and doesn't require
a redundant pipeline barrier.
Since our render passes are static and don't change throughout the
lifetime of a ra_renderpass, we unfortunately don't have much
flexibility here - so just hard-code SHADER_READ_ONLY_OPTIMAL as the
output format as this will be the most common case.
We also can't short-circuit the transition when we need to preserve the
framebuffer contents, since that depends on the current layout; so we
still use an explicit tex_barrier in this case. (Most optimal for this
scenario would be an input attachment anyway)
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