Performance Enhancement: Font rendering in a single drawcall

[Vwing]

Both SDF and classic fonts were being rendered a single character at a time, causing potentially tens of thousands of draw calls in a single frame. I've reduced it down to a single draw call per text item.

Before (18k draw calls and 30 fps):

After (500 draw calls and 60 fps):

The old multi-call path is still present and called if and when an error ever occurs in the new path.

[Vwing]

Note the drawcalls could be reduced further in books by batching lines together, and I believe I'm doing that in the Android fork, but it might cause some appearance differences. Will examine later, but it's a lower priority; I think this PR is fine without that addition.