iTermMetalRenderer.h

Implement a really damn elegant solution to subpixel antialiasing in the GPU renderer, if I don't say so myself.

- Add a frame cursor renderer. Pass screen scale through to all transient states.
- Use a single value for gridsize between frameData and perFrameState
- Subpixel color channels are completely independent. Change models to be keyed on a pair of colors, not a combo of six.  Get bilinear interpolation working! YAY!
- Refactor here and there
- Move inputs to transient state into a new class so it's easier to pass new data to renderers' transient states
- Fork the text fragment shader into a blending and nonblending version
- Draw background image to a temporary texture so the text renderer can sample its background color to do subpixel aa
- Alpha blend default background color over background image