ENH: Allow for specifying number of samples

In concert with https://github.com/commontk/CTK/pull/483, this enables specifying the number of multisamples (e.g., for systems where auto-detection fails).
Instead what about:
```
this->FSAAComboBox->addItem("Off", 0);
this->FSAAComboBox->addItem("Auto", -1);
this->FSAAComboBox->addItem("2x", 2);
this->FSAAComboBox->addItem("4x", 4);
this->FSAAComboBox->addItem("8x", 8);
this->FSAAComboBox->addItem("16x", 16);
```