gles2n64: Fix big regression - should not clear color buffer.

in OGL_ClearDepthBuffer function - fixes Turok 1 (graphics were
just black before), fixes Banjo Kazooie (whenever HUD appeared,
screen turned black - fixed now), fixes Waverace 64 (intermission
screens had a black background), and more

[cherry-pick from https://github.com/libretro/mupen64plus-libretro/commit/2b57b5183941762e12fffcd840ef47680eb5592b]