Added ability to create dynamic inputs to the audio gain plugin,
so it can be used for mixing in audio graphs.
Updated the Mesh class with support for specifying the maximum
number of primitives to draw and added two new plugins to make
use of the new features:

Mesh get primitive count: Get the current number of primitives in
a given mesh, and:

Mesh set max primitive count: Set the maximum number of primitives
to draw. -1 = all. The supplied value is clamped to the legal
range.