Get meshimage working properly in CairoMakie by overriding it

[asinghvi17]

Meshimage could be made to work properly for any transformation, if we created a bad mockup of a triangle rasterizer. Alternatively we know that the meshimage must be drawn per pixel, so could pseudo-rasterize a map of pixel positions to original positions.

This would probably only be for (a) PNG output or (b) if rasterize=true. SVG and PDF could either use the rasterization approach with 3x resolution, or just use the direct mesh as they do now. That's up for debate!