GL_EXT_multisampled_render_to_texture allows singlesampled textures to
be used with multisampled framebuffers in such a way that the final
resolve operation is automatically done.  In Vulkan terminology, the
render-to-texture GL attachment is used as a Vulkan subpass resolve
attachment with an implicit (ideally-)lazy-memory multisampled image as
the color attachment.

This extension expects that if the texture is drawn to after the
automatic resolve, the implicit multisampled image would take its
fragment colors from the singlesampled image.  In other words, the
opposite of a resolve operation should be automatically performed at the
start of the render pass.  This change refers to this operation as
"unresolve".

The goal of this extension is to allow tiling GPUs to always keep
multisampled data on tile memory and only ever load/store singlesampled
data.  The latter is achieved by using a subpass resolve attachment and
setting storeOp of the multisampled color attachment to DONT_CARE.  This
change achieves the former by using an initial subpass that uses the
resolve attachment as input attachment, draws into the multisampled
color attachment and sets loadOp of said attachment to DONT_CARE.

Bug: angleproject:4881
Change-Id: I99f410530365963567c77a7d62fc9db1500e5e3e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2397206
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>