glamor: Only use dual blending with GLSL >= 1.30

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

Closes: xorg/xserver#97
Fixes: e7308b6c "glamor: Add support for CA rendering in a single pass."
Reviewed-by: Dave Airlie <>
Reviewed-by: Kenneth Graunke <>
(Cherry picked from commit 5bfca003)
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