Add support for GL_ARB_texture_border_clamp and GL_ARB_texture_mirrored_repeat,

Add support for GL_ARB_texture_border_clamp and GL_ARB_texture_mirrored_repeat,
tested using texwrap, fixing the sourcing of border constant values (use the
bytes, not the floats!).  Remove the fallback on GL_CLAMP_TO_EDGE, which acts
properly, and add a note to GL_CLAMP about nonconformance (whcih is what the
fallback for CLAMP_TO_EDGE was probably targeting).