GTK3 issues

[neurolabusc]

The upcoming release of MRIcroGL for Linux will target the GTK2 and QT5 widget sets. The following (fixable) issues remain:

• There are autosizing issues and some controls are frozen until the screen is resized.

There are inherent limitations with GTK3 that make it poorly suited for this project. Since GTK 3.22 was supposed to be the last version, these are unlikely to be resolved.

• OpenGL [multi-sampling is not supported](https://github.com/aklomp
  /gtk3-opengl/issues/2), therefore the quality of MRIcroGL images in GTK3 is poorer than QT5 and GTK2.
• Complicated GTK3 widgets like GtkSpinButton never have real GtkWidget children. This creates limitations inusing these widgets with MRIcroGL.

Therefore, while one can compile MRIcroGL for GTK3, the functionality is limited.