Add MESA_image_sRGB spec.
dri: Add another duplicateImage to DRIimageExtension

duplicateImage will allow you to create a linear or sRGB view into a
DRIimage you have access to. This is useful because compositors may want
a specific view which doesn't correspond to the one used by
applications.
gallium: Implement DRIimageExtension.duplicateImage
egl: Add MESA_image_sRGB extension.

This gives applications access to use DRIimage.duplicateImage to create
sRGB and linear views from EGL images.