mesa/core: Add definitions and translations for EXT_texture_sRGB_R8

mesa/core: Add definitions and translations for EXT_texture_sRGB_R8

v2: - fix format definition line
    - disable  for desktop GL
    - don't add GL_R8_EXT to glext.h since it is already in
      GLES2/gl2ext.h in glext.h and include this header  where needed
      (all Emil)
v3: - swrast: Fill the function table for sRGB_R8
      The size of the function table is checked at compile time and must
      correspond to the number of mesa texture formats.
      dri/swrast being gles-2.0 doesn't support the extension though
v4: - correct format layout comment (Ilia Mirkin)
    - correct logic for accepting GL_RED only textures (in part Ilia Mirkin)
      EXT_texture_sRGB_R8 requires OpenGL ES 3.0 which includes
      ARB_texture_rg/EXT_texture_rg, so one only must check for the first
      when SR8_EXT is really requested.
v5: - add define for GL_ES8_XT to glheader.h and don't include GLES
      headers  (Ilia Mirkin)

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>