Simplify `QPainter::drawRoundedRect` usage

After fixing the porting mistake done in 78d983ee, `drawRoundedRect` can
be further simplified to not use any relative sizes anymore.

We can also assume that the code in `dviRenderer::epsf_special` was
supposed to use a constant corner radius and therefore was wrong
initially. By dropping `Qt::RelativeSize` the code might become closer
to its originally intended behavior.

Test Plan:
  - No change in behavior for rounded corners of "Welcome" popup message
    and "No Annotations" message in sidebar.
  - Could not test DVI code path.