Make GraphicsContectGL ANGLE implementation easier to edit correctly
Currently the header file is shared by three unrelated implementations. It is relatively hard to add code to the header file.
Currently the implementation is distributed to many (four?) implementation files. It is relatively hard to remember to edit all files.
Currently the distinct implementations have duplicated, non-shared code for lot of features that should share the code.
This is a top level bug to explain the depending bugs.