Texture "types" are the same as texture "targets" except for cube maps.
Cube map targets specify a single face. Cube map types specify a whole
cube map. The subImage functions should take a target instead of a
type. We were using both in different places. This CL corrects all uses
in subImage calls to "target". It also adds a helper for getting a
target texture from a target. And clarifies the naming of the texture
query methods.

Bug: angleproject:3356
Change-Id: I06eb5c5666eec9b8934becf2ba57a066d5cdabde
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1558672
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>