The main issue that gets fixed here are:
- VK_IMAGE_LAYOUT_UNDEFINED cannot be used if the image content is to
be preserved after the transition. Unfortunately, there is more than a
few places in the VK backend where we use VK_IMAGE_LAYOUT_UNDEFINED on
images that are later used as sources.
- accessing an image memory from the host is only supported if the the
layout is PREINITIALIZED or GENERAL. However, images in the staging
pool were transitioned to other layouts and put back into the pool,
later they would be mapped/memcpy'ed into. Instead we now always use
the GENERAL layout and never change it.
- images that where in the pool where transitioned using the miplevel
of the texture we were copying into, however this didn't make sense
because images from the pool don't have mip levels.
As part of these fixes, we also refactor imageLayoutTranstion(),
because there was two copies of this function, one specific to texture
and another one more generic. We now always use the more generic one,
with a new helper for textures.
Note: didn't fix "readPixels" which has at least problem (1), because
it's a tricky implementation, and it requires more toughts.
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