Not as straightforward as it sounds, but not hard either:
* Read OS/2 sfnt tables from TrueType fonts
* Calculate strikethrough position/thickness (prefer font-advertised if possible, calculate if not)
* Plumb the SGR code through the terminal state -- does not increase cell memory size
* Modify the shader to support it
The shaders are getting pretty nasty after this... there's tons of room for improvement. I chose to follow the existing shader style for this to keep it straightforward but will likely soon refactor the shaders.
