GetLoadFunctionsMap() is expected to cover all possible combinations of
texture internalformat and upload data format.  Several of these
combinations were missing, especially w.r.t to compressed types.  This
change adds the missing combinations that are hit on any of the
configurations being tested.  An assert is added to catch missing
conversion functions in the future.

Bug: angleproject:2670
Change-Id: I793ac2c57f65eeb1cc8da8f1b223f6a9a44c7708
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1524462Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>