added support for a shuffle key in  the [music] music to allow selecting between random and non-random music play
added modifications to changelog
shuffle sides based on the settings value

Before, it was based on the preference value. This was a problem when not using the mp_configure dialog (sp)
Merge pull request #265 from Zappaman/music_shuffle_feature_2

add option to not shuffle music, see https://gna.org/bugs/?19653
Add submerge and color modulation shader attributes.
Don't limit color modulation to the [-128, 127] range.
Properly set shader attributes in rectangle draw functions.
Fix sdl::timage copy ctor.
Consistent naming pattern for shader variables.
Exception for shader errors.
Add an exception for SDL_gpu errors.
Implement submerging in SDL_gpu builds.
Implement vertical and horizontal flip and grayscale effects.

Raises required GLSL version to 1.30.
Better SDL_gpu implementation for display::render_image.
Fix effects shader attribute.
'varying' -> 'in' or 'out' in shader sources.
Fix floating labels for SDL_gpu.
Don't print fps rate to cerr in SDL_gpu builds.

Now that floating labels are working, it's no longer necessary.
Draw map background in SDL_gpu builds.

Not yet perfect as the texture refuses to repeat vertically. Not sure
why.
Fix fullscreen mode for SDL_gpu.