kmscube: gears: use only GLES2

The gears mode currently depends on either GLES3 or desktop GL to build, but in both cases without a proper meson dependency check. It also doesn't really use any features which would require those GL versions over GLES2, which is already a global dependency of kmscube. So just change it to use symbols which are available in GLES2 and it can be built and run in all platforms.

This is an alternative fix for !51 (closed) which I suggested in !51 (comment 2378944)

Closes: !51 (closed)