Renames 'angleFormat' to 'intendedFormat'. Also renames 'bufferFormat'
and 'imageFormat' to 'actualImageFormat' and 'actualBufferFormat'. This
renaming should make it clearer to the reader what the meaning of the
different format fields are. Intended format is the front-end format
and the actual formats are the formats we pass to Vulkan. Also updates
the documentation.

Bug: angleproject:4009
Change-Id: If61bf7250e88f7ed3d452718574c963d718e27b2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1866077
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>