BlitGL: Fix LUMAWorkaround for half-float copies Use desktop HALF_FLOAT enum when running on desktop OpenGL. Bug: angleproject:5038 Change-Id: I829f28ca51caa1f98b653d1a456ed775f8d362cb Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3738261 Reviewed-by: Kenneth Russell <kbr@chromium.org> Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com>