Add background color drawing support

[ncallaway]

This would involve adding support for a background color in text area, which would fill the Text Bounds with the background color, and then alpha blend the text onto the background color.

This would allow me to move most of my text rendering from my transparent draw calls into my opaque draw calls (basically, anytime text appears on a solid background color).

I think this would mostly involve changes to the TextArea struct (adding something like a background_color as an Option<Color>, the shader to be able to render the background color, and the glue between those two elements.

[ncallaway]

Just saw: #55. I'll go ahead and close this, since it's really not a problem to manage it myself