don't delete any gl resources in dispose
Merge pull request #2425 from jdf/master

Fix NPE in PDE, affecting color picker and other Tools.
getBufferPixels() removes the buffers from the cache, fixes the #2424
some cleaner logic to allow for #2424, which should also work when video
buffers start accumulating in the cache on slower machines.
override keyword loading for Python
slightly more informative update message in console (to differentiate from version)
additional items of note
Merge with upstream.