GL requires that imageAtomicExchange be supported for r32f formats.
However VK_FORMAT_FEATURE_STORAGE_*_ATOMIC_BIT is nearly unsupported
everywhere without some Vulkan extension that brings in unnecessary
support.

This GL feature is emulated by transforming the shader to use r32ui for
all images that originally specified r32f.  floatToUintBits and
uintBitsToFloat is used to maintain correct usage of the image* builtin
functions.

Bug: angleproject:5535
Change-Id: Ie607089935d3283b3ffa054f4b4385b81fb8f53d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2635453
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>