draw basic checkerboard
cycle through screens with arrows and hold grid for 4 minutes maximum

Apple seems to use a black and white checkerboard displayed for 4 minutes then displays a black screen as their means to evaluate image retention in the panel - might as well copy that.
add simple 4 minute countdown on the grid
regenerate grid texture on display change

The grid texture is generated at the display size, I couldn't figure out how to get GL_REPEAT working properly.
fix bad time countdown calculation