The RLE TGA images were converted from raw TGA ones using GIMP and
some tweaks as the GIMP exporter only support bottom-left and top-left:

- Left ones are unmodified GIMP export.
- Right ones are in-GIMP horizontally flipped images exported as left
  ones then the TGA X flip bit was modified by hand in a hex editor.

The TGA format is a bottom-left format by default: the first byte
of the first column is expected to be displayed at the bottom-left
of the screen, this behavior is inherited from devices painting the
screen from the bottom left to the top right.

The TGA format provides two bits in the image descriptor byte to paint
the data from other sides. The 4th bit tells the device to paint the
screen from right-to left and the 5th bit tells the device to paint
the screen from top to bottom.

So basically:

- 00: from bottom-left to top-right
- 01: from top-left to bottom-right
- 10: from bottom-right to top-left
- 11: from top-right to bottom-left

Previously stb_image only read the 5th bit and then only supported the
loading of vertically flipped images, stb_image was ignoring the 4th bit
coding the horizontal flip. Now both flipping directions are supported
for both raw and RLE storage.

- appveyor: do not build the shared library (it is unusable yet),
- azure-pipelines: make all binaries use the same library,
- codeql: use the shared library.