Save walls using TPT++ wall numbers

Also TPT++ jacksonmj 02a4c17d: Ignore invalid walls in some old saves, so that they load correctly
TPT++ Simon: spark signs
TPT++ 12261e26: make Ctrl+X delete stacked particles
TPT++ Simon 2d22776a: Iterate through signs in reverse order when seeking for click events in order to ensure when clicking on stacked signs the top-most sign is detected
TPT++ jacksonmj 186f8a17: No more red DEUT
TPT++ Simon 6bff3b56: Don't open saves larger than 200MB
TPT++ mniip 30c80220: fix PIXR() macro: if MSB of the color is nonzero, it used to return a >255 value, which, say, in Renderer.cpp would get clamped to 255, no matter what the actual red byte is
TPT++ jacob1 78c89dc6: fix crash with LIFE