Severe performance improvement for brush preview rendering. Fixes #111.

[Henry00IS]

Issue #111 Found a huge potential performance optimization explains in detail what the problem is.

Instead of creating proper indices on the CPU I now immediately push duplicate vertices to the GPU. I also replaced all of the C# lists with fixed-size arrays reducing the memory usage per call from 1.5 GiB or more to an insignificant amount that no longer freezes the editor.

Performance improvements when:

• Moving complex brushes around the scene.
• Disabling/Enabling brushes.
• Viewing excluded polygons.
• Loading scenes.
• Entering play mode (this behavior has yet to be fixed but perhaps Shouldn't all brushes use the "EditorOnly" tag? #113 is a good option).

Attempting to move a single extremely complex brush with 512 polygons, before:

Using this new pull request:

Attempting to view excluded polygons, before:

Attempting to view excluded polygons, after: