compositor: add support for OES_EGL_image_external

compositor: add support for OES_EGL_image_external

In cases where the GPU can natively handle certain YUV formats,
eglQueryWaylandBufferWL() can return the value EGL_TEXTURE_EXTERNAL_WL
and the compositor will treat the buffer as a single egl-image-external.

See:
http://www.khronos.org/registry/gles/extensions/OES/OES_EGL_image_external.txt

v1: original
v2: rename EGL_TEXTURE_EXTERNAL_OES -> EGL_TEXTURE_EXTERNAL_WL and query
    for the extension
v3: fix build without updated mesa headers, if EGL_TEXTURE_EXTERNAL_WL
    #define is missing from older mesa headers.
v4: resend without missing parts

Signed-off-by: Rob Clark <rob@ti.com>