Fix for memory leak in previous commit
Volumetric strokes are also drawn during stroke drawing too

To make it easier to see the final effect of volumetric strokes, these are now
also drawn when painting new strokes
Code cleanup: Tidy up some stuff from previous commit
Fix: Poll for toolshelf UI buttons was spitting out warnings

Casting to bool is safer there
Make volumetric stroke drawing a per-layer option
Volumetric strokes draw in 2D views too now
Code cleanup: renamed function for drawing 2D strokes to make more sense now
RNA: Line width for volumetric strokes is allowed to be higher

While 3D strokes currently have a size limitation, this doesn't apply to volumetrics.

TODO
Most of our 2D strokes don't have a limitation either, but we can't easily
identify such cases in this function, so for now the limit holds.
GPencil Drawing: Support for drawing filled strokes

It is now possible for the insides (i.e. the region contained within the stroke)
Grease Pencil strokes to be drawn "filled". This initial implementation is quite
crude.

Main Limitations:
- It uses OpenGL polygons for rendering. The main thing to be aware of with this
  is that it *DOES NOT HANDLE CONCAVE* strokes too well. That is, it will just
  plaster over the concave regions, which may not be what you really want

- Fills are currently turned on/off using the opacity settings. Whether we continue
  doing this remains to be seen...