gsk: Use some GLES extensions

Allow our shaders to use samplerExternalOES, by declaring
that we use the relevant extension. Unfortunately, this
only works for gles, and requires different extensions for
gles2 and gles3. Yay

gsk/gl/resources/preamble.glsl

@@ -1,3 +1,9 @@
+#if defined(GSK_GLES3)
+#extension GL_OES_EGL_image_external_essl3 : require
+#elif defined (GSK_GLES)
+#extension GL_OES_EGL_image_external : require
+#endif
+
 #ifndef GSK_LEGACY
 precision highp float;
 #endif