(50G) [HPGCC3] DRAW3DMATRIX replacement with grayscale surfaces, proof of concept

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Well, that's definitely weird. I copied your hellofix.c code into a new project, compiled it, loaded it into my x49gp instance, and it simply worked. I'm attaching the build output in case you want to test it on your installation.
The example you labeled as "working" is using the black-and-white mode - are you using the grayscale-enabled version of x49gp? If not, that would explain why the other examples don't look right. (They would look right on a real calculator.)
I patched x49gp a few years ago, and the patch has been accepted by x49gp's author pretty quickly. I recommend cloning your x49gp directory from that repository, or perhaps Claudio's fork if his command line switch for debugging looks appealing.