FIX debug bitmap should not be free'd

[ninovanhooff]

⚠️ I'm out of my depth here

Please review carefully, but I think this is the way to do it

Before this change
Simulator would crash with a nillegal access exception on the second invocation of this sample

let debugBitmap = gfx.getDebugBitmap()
gfx.pushContext(debugBitmap)
gfx.fillRect(x, y, width, height, kColorWhite)
gfx.popContext()

After this change
🌞

[Nycto]

Does this repro if you put it in a test?

[ninovanhooff]

Yes. Though on Mac the Simulator freezes indefinetly if the test is broken.
Either on linux this doesn't happen, or the github runner will time out, which would also make the test fail. It would then be hard to trace that fail back to this test tho.

Added a test to t_ghraphics

By the way, if you could add a little manual to README.md on how to run the tests locally, that would be appreciated. I used the very crude

 rm ./stderr.txt && rm ./stdout.txt && nimble simulator && open --stdout ./stdout.txt --stderr ./stderr.txt ./tests.pdx

Inside the tests folder.