On iOS, GL_OES_depth_texture is supported, which includes the format
GL_DEPTH_COMPONENT32_OES. However, GL_OES_depth32 is not always exposed.
This is causing ANGLE to not expose GL_OES_depth_texture, and
GL_ANGLE_depth_texture.

Bug: angleproject:4451
Change-Id: If9cd782e851e040b1cf24a763dd93e063ebffbc7
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2095361Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>