Code Cleanup: Remove some redundant operations

Code Cleanup: bool in place of short

Fix: GP Stroke verts are now properly masked behind geometry now when X-Ray option is disabled

Code Cleanup: Deduplicate logic for calculating 2D stroke coordinates for drawing

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...

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.

Code cleanup: renamed function for drawing 2D strokes to make more sense now

Volumetric strokes draw in 2D views too now

Make volumetric stroke drawing a per-layer option

Fix: Poll for toolshelf UI buttons was spitting out warnings

Casting to bool is safer there

Code cleanup: Tidy up some stuff from 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

Fix for memory leak in previous commit

Proper implementation for "Volumetric Strokes" drawing

Due to the enthusiastic reaction from the community, I've decided to make this
a proper drawing technique. This commit separates the drawing code for this
out into its own method, and reduces the amount of costly GL read/write calls used
by performing more of this using Blender's internal math library. As a result,
it seems (at least in my limited tests) to be running much faster :)

TODO:
- Configuration options for controlling when this is used in combination
with the other drawing techniques
- Use for stroke drawing too when enabled?
- Allow setting stroke sizes to be much thicker (and also set a better base-size)

Hack: Quadrics-based drawing technique for 3D strokes

This is really really hacky stuff. I've literally just dumped this code in there
without any attempt at cleaning it up or properly integrating it, as it was just
a quick test which turned out to provide some rather interesting results!

UI and properties support for having "Filled" GPencil Strokes/Shapes

GPencil Draw Operator: Implemented proper method to delay start of drawing

When running GPencil drawing operators from the toolbar or from a (pie) menu,
drawing operations shouldn't start immediately, but rather, only when users
actually click to start drawing. This is because the mouse is often not in a
suitable location for this to work when the operators get invoked from such
UI controls.

Previously, we used a hack which just detected certain types of events. This worked
ok for toolbars, but not for menus. Now, we've got a proper property to enable this
behaviour instead.

Expose Grease Pencil "Convert to Curves" operator directly in toolbar instead

It only really applies to the 3D view, so this makes more sense, and fits
the workflows for modellers and mograph artists more.

Tweaks for Delete Active Frame in GPencil UI

* Expose "delete active frame" directly in GP UI (alongside frame locking)
instead of hiding in a submenu
* Deleting the active frame immediately makes the one before it active

Bugfix: Fix for potentially uninitialised matrix, and 2D eraser being broken following recent changes