Use the magic behaviour of GL_BACK in GLES 1 and 2 as well as 3

In GLES 3 it is not possible to select rendering to the front buffer and
instead selecting GL_BACK has the magic interpretation that it is either the
front buffer on single-buffered configs or the back buffer on double-buffered.
GLES 1 and 2 have no way of selecting the draw buffer at all. In that case we
were initialising the draw buffer to either GL_FRONT or GL_BACK depending on
the context's config and then leaving it at that.

When we switch to having configless contexts we ideally want Mesa to
automatically switch between the front and back buffer whenever a double-
or single-buffered surface is bound. To make this happen we can just allow
the magic behaviour from GLES 3 in GLES 1 and 2 as well. It shouldn't matter
what the internal value of the draw buffer is in GLES 1 and 2 because there
is no way to query it from the external API.

Reviewed-by: Kristian Høgsberg <[email protected]>

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