Move text rendering core to SdlSurfaceText.

This separates the actual text rendering from the surface management so
we can render text onto arbitrary surfaces and manage them differently
depending on the usage.
Move line-height calc to SdlSurfaceText.

This removes all of the text rendering out of SdlLabelView.
Add SdlSurfaceText ctor with font and color.

The common case is that at least the font (and maybe the color) will be
initially set, so provide a constructor for that case.