Add min_bpp setting in xrdp.ini

[matt335672]

This discussion is prompted by issue #2240 - thanks to @EntityinArray for raising it.

We've got a max_bpp setting in xrdp.ini, but no min_bpp. From my reading of section 2.2.1.3.2 of [MS-RDPBCGR] it should be possible to add this.

What do people think? Having a minimum color depth of 24 by default would seem to be a good idea on modern systems. A lot of features like GLX won't even work with 16-bit colour. Users who have 16 bpp use-cases can change this setting to 16, or even remove it.