Blender Git Loki

Cleanup and improvements of the compositor debug output.

Debug code for graphviz output moved to a dedicated file COM_Debug.h/cpp.

The DebugInfo class has only static functions, which are called from a number of places to keep track of what is happening in the compositor. If debugging is disabled these are just inline stubs, so we
don't need #ifdefs everywhere and don't get any overhead.

The graphviz output is much more useful now. DebugInfo keeps track of node names in a static string map for meaningful names. It uses a number of colors for various special operation classes.
ExecutionGroups are indicated in graphviz with clusters.

Currently the graphviz .dot files are stored in the BLI_temporary_dir() folder. A separate dot file is generated for each stage of the ExecutionGroup scheduling, this is intended to give some idea of the
compositor progress, but could still be improved.