gl-renderer: require GL ES 3.0 for color management

Trying to support GL ES 2.0 + extensions along with GL ES 3.0 for better
control is becoming too complicated fast. In this patch you see the
GL_RGBA vs. GL_RBA16F and GL_HALF_FLOAT vs. GL_HALF_FLOAT_OES paths.
More such cases will come, e.g. GL_RED_EXT vs. GL_R32F.

Make things simpler and require GL ES 3.0 +
GL_EXT_color_buffer_half_float for all color management related
functionality. If one doesn't have GL ES 3.0, all you lose is color
management.

Also, all extensions needed by color transformation operations are
gathered under one boolean flag instead of having a flag per extension,
again for simplicity.

This makes the GL ES extension handling much easier.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>