Swiftshader's depth support requires that ANGLE fall back to using
the D32FS8X24 depth format. Some conversion routines needed to be
added to support that use.

Test:
dEQP.GLES3/functional_texture_format_sized_*

Bug: angleproject/3937
Change-Id: I979dc5e9b01dac40f564daad4f4817b61bd056ac
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1829170
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>