gl-renderer: Use highest precision for vertex shaders
requested to merge daniels/weston:gl-renderer-highp-hopefully-most-of-the-time-but-not-always into main
Whilst GLSL requires highp for the vertex shader stage, highp is optional for the fragment shader.
Make sure that we work in highp by default during the vertex shader stage, using either highp or mediump for the texcoord varying according to what the fragment shader supports.
Signed-off-by: Daniel Stone daniels@collabora.com