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Partially implement feature request #129:
BITLOAD and Bitmap -> Load... can now load JPEG image files as an undocumented feature.
The implementation uses wxWidgets to load the JPEG, which uses libjpeg from the IJG.
The feature is incomplete. It should also be able to load PNG and GIF, have some good tests for all supported file formats, have translations in the file picker for Bitmap -> Load... and be documented.

Fix a recent (unreleased) regression that caused BITLOADSIZE to load the bitmap.
This was introduced by recent refactoring which didn't account for an undocumented and subtle calling convention to instruct OpenDIB not to draw the bitmap to the screen.
I documented this calling convention and added a regression test to help prevent this from happening again.
I make it obvious (not subtle) in the future.

Partially implement feature request #129:
BITLOAD and Bitmap -> Load... can now load JPEG image files as an undocumented feature.
The implementation uses wxWidgets to load the JPEG, which uses libjpeg from the IJG.
The feature is incomplete. It should also be able to load PNG and GIF, have some good tests for all supported file formats, have translations in the file picker for Bitmap -> Load... and be documented.

Add a comment to LL speculating on its history and purpose.

Add some regression tests that BITLOAD loads small bitmaps at the turtle's position and large ones at 0,0 window coordinates.

Add some bitmaps that can be used to test that BITLOAD puts large images in the upper left corner and small images at the turtle.

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.