Blender Git Loki

Fix #35884: crash opening .blend with generated color grid image and preview render.

Printing text on the color grid image would initialize font glyphs from a thread at
the same time as the UI, causing conflicts. The freetype glyph renderer needs to be
mutex locked because it uses a shared buffer internally even when rendering for
different fonts. Also needed to change the image generate function to use the render
monospace font to avoid conflicts in blenfont.

What's still weak in the blenfont API is that there is no distinction between a font
and a thread using that font to render with some particular size, style, etc.