Swiftshader does not have support for D24_S8 (typical fallback for
32bit DEPTH_COMPONENTS) but does have D32_FLOAT, so add fallback to
that format. This then allows ANGLE to support OES_depth_texture
extension when using Swiftshader which is needed to allow Vulkan
swiftshader to replace GL swiftshader.

Bug: angleproject:4232
Change-Id: Iaf04fcf0bea8d9cca0b9e7a18e3351ce8c704355
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1986413
Commit-Queue: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>