egl-wayland: Make sure we allocate a back buffer even if nothing was rendered

At eglSwapBuffer time, we blindly assume we have a back buffer, but the
back buffer only gets allocated when somebody tries to render something.

NOTE: This is a candidate for the 9.0 and 9.1 branches.

https://bugs.freedesktop.org/show_bug.cgi?id=60086
glsl: Fix unsupported version error for GLSL ES 3.00, future proof for 3.30.

When the user specifies an unsupported GLSL version,
_mesa_glsl_parse_state::process_version_directive() nicely gives them
an error message telling them which GLSL versions are supported.
Previous to this patch, the logic for determining whether a given
language version was supported was independent from the logic to
generate this error message string; as a result, we had a bug where
GLSL 3.00 would never be listed in the error message as an available
language version, even if it was really available.

To make matters worse, the code for generating the error message
string assumed that desktop GL versions were always separated by 0.10,
an assumption that will be wrong as soon as we support GLSL 3.30.

This patch fixes both problems by adding a table of supported GLSL
versions to _mesa_glsl_parse_state; this table is used both to
generate the error message and to check whether a given version is
supported.
WIP: don't allow non-flat varying structs containing integral types.

May also fix problems with varying arrays of non-ints, need to check for sure.

I don't want to send out this patch until we've resolved the question
of whether Mesa should be checking this on vertex shader outputs,
fragment shader inputs, or both.