Add extension stubs * GL_EXT_conservative_depth * GL_EXT_depth_clamp * GL_EXT_render_snorm Bug: angleproject:8046 Bug: angleproject:8047 Bug: angleproject:8048 Change-Id: I7deb4f25f76008103c2754747db2d90be880b6ca Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4296803 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>