ensure texture allocated in _get_gl_texture() in preparation

The plan is to defer a lot more work in creating a texture until
allocation time. This means we wont be able to assume that all textures
being used to render must have already been allocated when data was
specified.

The latest point at which we will generally require a texture to be
allocated will be when we need to know the underlying GL handle for a
texture and so this updates cogl_texture_get_gl_texture() to ensure the
texture is allocated.

Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 59f6fefc37524f492512a71b831760a218d9bb95)

cogl/cogl-texture.c

@@ -345,6 +345,9 @@ cogl_texture_get_gl_texture (CoglTexture *texture,
 			     GLuint *out_gl_handle,
 			     GLenum *out_gl_target)
 {
+  if (!texture->allocated)
+    cogl_texture_allocate (texture, NULL);
+
   return texture->vtable->get_gl_texture (texture,
                                           out_gl_handle, out_gl_target);
 }