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>
diff --git a/src/weston-egl-ext.h b/src/weston-egl-ext.h
index 8e132c0..1d4b76b 100644
--- a/src/weston-egl-ext.h
+++ b/src/weston-egl-ext.h
@@ -39,6 +39,7 @@
 #define EGL_TEXTURE_Y_U_V_WL            0x31D7
 #define EGL_TEXTURE_Y_UV_WL             0x31D8
 #define EGL_TEXTURE_Y_XUXV_WL           0x31D9
+#define EGL_TEXTURE_EXTERNAL_WL         0x31DA
 
 struct wl_display;
 struct wl_buffer;
@@ -53,4 +54,8 @@
 
 #endif
 
+#ifndef EGL_TEXTURE_EXTERNAL_WL
+#define EGL_TEXTURE_EXTERNAL_WL         0x31DA
+#endif
+
 #endif