evas: evas_gl - Got rid of resource pool in favor of creation on demand.

evas: evas_gl - Got rid of resource pool in favor of creation on demand.

Resource contexts/surfaces are used for creating resources within Evas_GL.
In oder to handle Evas_GL runnig from different thread than the main one,
a resource context/surface pool was used. This turned out to be unnecssary
as they are not used very frequently.  So, I got rid of the pool and
made the resources create as needed.