glamor: Only use dual blending with GLSL >= 1.30

Merged Michel Dänzer requested to merge daenzer/xserver:glamor-dual_blend-glsl-1.30 into master

It can't be used with older GLSL. Fixes a crash when attempting to anyway.

Closes: #97 (closed) Fixes: e7308b6c "glamor: Add support for CA rendering in a single pass."

/cc @airlied

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