Respect opacity when drawing annotations

[Carifio24]

This PR resolves #237, allowing the engine to respect the opacity value of annotations when drawing them.

The implementation here is meant to work with the way that the engine handles drawing annotations (which I hadn't really understood when I opened the original issue). Basically, the primitives for each annotation don't get passed into the relevant shader(s) separately - what happens is that they get added to global collections of the relevant primitive type ("global" here meaning static member values of the Annotation base class). Then the shader calls for each "list of primitives" type are made inside of the drawing methods in DrawBatch.

What this means is that we can't use the opacity argument of each annotation in the Draw/DrawLines calls (e.g. here and similar calls), because these calls are made in a static method without access to a particular annotation's opacity. Thus, I think the only good way to do this is to inject the opacity into the passed color - basically, if an annotation has color (A,R,G,B) and opacity α, we pass the shader (A*α,R,G,B). This is the approach taken here. Note that I had to round the values for this to work correctly - Color.A is typed as a float, but only seemed to work when I gave it an integer.

For testing this, I used the example code I gave in #237, plus some similar code for Poly and Circle annotations.