UI and Font improvements

Several UI and Font changes:
- Improved rendering quality for fonts generated from gameplay-encoder.
- Completely removed off-screen rendering of forms. All forms are now rendered immediate mode to the currently bound framebuffer. This reduces the overhead of managing potentially multiple offscreen framebuffers and also fixes some "double blending" issues that were causing incorrect blending of images and fonts for UI.
- UI drawing is now batched by default, reducing the number of draw calls for forms down to only a couple, from tens to hundreds previously. A form property "batchingEnabled" has been added to allow turning off batching under rare cases where batching introduces rendering glitches (mainly for complex forms with multiple layers of semi-transparent controls and text).
- Added support for default themes (explicitly specifying theme and style for forms is no longer required). A new ui\defaultTheme property is now supported in game.config to specify the default theme for a game.
- Various bugfixes and improvements.