add tests to verify render output

[christianparpart]

I have found this: https://github.com/marketplace/actions/setup-opengl

With that i can add PNG screenshot support as VT sequence and then implement tests to verify render output.

Tests could cover the following rendering tests:

• sixel image
• emoji variants
• ligatures
• glyph positioning for known problematic groups
• SGR decorations
• contour: input mappings (shortcuts)
• contour: live config change
• contour: Special keyboard layouts

Additional action items

• VT sequence: screenshot #161 VT sequence to take screenshot (png and maybe BMP or TGA for easier pixel testing?) at a given cell region (default: so grid cells, main page area)
• VT sequence to set font #162 VT sequence to set current font. OSC 50 ; Pt ST
• test runner script that must also work locally for quick-testing during development; similar to isoltest
• test runner: if failure, it should be available if highlighting the regions that du not Match by blending a tint color into it and report hier many pixels out of how many in total whether affected. Also show expected and actual separately.
• test runner: interactive mode for updating expectations
• Add to Github Actions
• upload failed test results to artifact store. A test case result should contain all, expectation and actual and actual with highlighted regions side by side.