egl: add initial EGL_MESA_image_dma_buf_export v2.4

egl: add initial EGL_MESA_image_dma_buf_export v2.4

At the moment to get an EGL image to a dma-buf file descriptor,
you have to use EGL_MESA_drm_image, and then use libdrm to
convert this to a file descriptor.

This extension just provides an API modelled on EGL_MESA_drm_image,
to return a dma-buf file descriptor.

v2: update spec for new API proposal
add internal queries to get the fourcc back from intel driver.

v2.1: add gallium pieces.

v2.2: add offsets to spec and API, rename fd->fds, stride->strides
in API. rewrite spec a bit more, add some q/a

v2.3:
add modifiers to query interface and 64-bit type for that (Daniel Stone)
specifiy what happens to num fds vs num planes differences. (Chad Versace)

v2.4:
fix grammar (Daniel Stone)

Signed-off-by: Dave Airlie <airlied@redhat.com>