glamor: Use glsl "fract/mod" instead of "while" in gradient shaders.

This fixes gtkperf. It seemed to hang forever.
glamor: Use epoxy_has_gl_extension() instead of rolling our own.
glamor: Use epoxy_gl_version() instead of rolling our own.
glamor: Update GL requirements to 2.1.

We will never ever run on OpenGL 1.2 as we use shaders everywhere.
2.0 may be enough, but we also often use PBOs and our big shaders
won't fit into the first GLSL limits.
glamor: Select VBO path by ARB_mbr extension.

The mbr path was hard coded enabled for desktop gl and disabled for gles.
But there are both, desktop without mbr and mobiles with mbr.
glamor: Don't reset GL_ELEMENT_ARRAY_BUFFER as it's always bound to the same buffer.
glamor: Remove unneeded unbindings.

They are already cleared on glamor_put_vbo_space.
glamor: Use vbo for solid boxes.
glamor: Drop feature dependent optimization on startup.

We don't care that much about startup time to write different code paths...