CoreText Renderer is slow under MacVim release 176

[ychin]

Release 176 contained a fix for misc CoreText renderer bugs and glitches (#1356), and the fix for clipping is causing some performance regressions. You can tell this is happening just by scrolling.

We should fix the bug. Not getting clipped text is important but it shouldn't have caused the regression.

Meanwhile, if users are encountering this, they can do defaults write org.vim.MacVim MMRendererClipToRow 1 to temporarily work around it (this just makes it work like before, where tall texts would get clipped), but they should unset it once the fix is in.

[ychin]

For some reason I couldn't reproduce this after a while. Seemed to be some weird state that macOS got into with the new build. I'm still not sure what happens and can't get it to show up again…