[cogl] Allow Cogl to create NPOT textures for GLES2

The core GLES2 API supports NPOT textures, i.e. there is no extension as for
OpenGL, so we now add COGL_FEATURE_TEXTURE_NPOT to the feature flags in
_cogl_features_init.

Thanks to Gordon Williams for spotting this.

gles/cogl.c

@@ -62,7 +62,8 @@ _cogl_features_init (void)
     flags |= COGL_FEATURE_FOUR_CLIP_PLANES;
 
 #ifdef HAVE_COGL_GLES2
-  flags |= COGL_FEATURE_SHADERS_GLSL | COGL_FEATURE_OFFSCREEN;
+  flags |= COGL_FEATURE_SHADERS_GLSL | COGL_FEATURE_OFFSCREEN
+    | COGL_FEATURE_TEXTURE_NPOT;
 #endif
 
   flags |= COGL_FEATURE_VBOS;