FMSLogo Code

Fix Bug #579; BITMAPTURTLE turtles are now drawn in correct position when zoomed.
This was fixed by using a backbuffer for both sprites and legacy bitmapped turtles (before, legacy bitmapped turtles were rendered without a backbuffer, which required special handling).
This will reduce the performance of programs that only used legacy bitmapped turtles.

Note that bug #24 is fixed.
Also note that loading some corrupt bitmaps may have different behavior.

Disable handling alpha channels in 32bpp bitmaps.
As far as I know, .bmp files do not have an alpha channel. What's more, this code prevented truncated 32-bpp bitmaps from being rendered in the same way that truncated 24bpp bitmaps are rendered.

Enable the "topdown" tests for 16BPP bitmaps.
These had been failing because due to a test bug (they were using the wrong file name). The images load correctly.

Fix a regression in CHECKSCREEN that used the wrong file extension for the per-computer reference data.

Update CHECKSRCREEN to be able to read expected values from "referencedata".
This makes setting up a new test machine easier, as the setup won't require manually approving as many expected data files.
This only became worthwhile after integrating bmptestsuite into the tests.

Cleanup: Remove three redundant tests (copy&paste error)

Increase the width,height limits on BMP from 2**15 to 2**16 in wxWidgets.
This enables FMSLogo to load more of the bitmaps from the bitmaptestsuite and has less risk of breaking programs that worked prior to using wxWidgets to load bitmaps.

Fix a recent regression where vbarred strings weren't unvbarred by DLLCALL when given as a "U" argument.

Shorten a comment: "well-known data" -> "known data"