Fixes to Slider and TexturePacker2

Slider:

Clamped value to min and max in setRange.
Allowed min to be same as max.
Added check, if min is the same as max, avoid division be zero.

TexturePacker2:

Added possibility to force the output texturepack to be square (can be set by boolean forceSquareOutput. My motivation is the following: in some cases mipmaps don't work unless the texture is square.

BTW: Sorry for making another pull request as I did not find out how to update the previous one.