app: fix "Result is not floating-point (UNINTENDED_INTEGER_DIVISION)".

See #3996.
This was a warning raised by the Coverity scan on one of the lines. I
fix also in the same time other arithmetics mixing int and double.
Better be thorough.
The specific warning was on:
> circ = 2.0 * G_PI * (private->width / 2)
where the division was integer, which was probably not intended. Of
course that error (display item only) was likely barely visible anyway,
still…
5 jobs for master in 25 minutes and 32 seconds (queued for 1 second)
Status Job ID Name Coverage
  Dependencies
passed #461063
deps-git

00:04:41

passed #461064
deps-x86_64-w64-mingw32

00:16:41

 
  Gimp
passed #461065
gimp-autotools

00:08:50

passed #461066
gimp-meson

00:06:06

failed #461067
gimp-x86_64-w64-mingw32-meson

00:02:14

 
Name Stage Failure
failed
gimp-x86_64-w64-mingw32-meson Gimp
error: Status code: 503 for https://mirrors.fedoraproject.org/metalink?repo=updates-released-f30&arch=x86_64 (https://mirrors.fedoraproject.org/metalink?repo=updates-released-f30&arch=x86_64).
Fedora 30 - x86_64 - Updates 8.5 kB/s | 3.4 kB 00:00
Cannot download 'https://mirrors.fedoraproject.org/metalink?repo=updates-released-f30&arch=x86_64': Cannot prepare internal mirrorlist: Status code: 503 for https://mirrors.fedoraproject.org/metalink?repo=updates-released-f30&arch=x86_64.
Failed to download metadata for repo 'updates'
Error: Failed to download metadata for repo 'updates'
Uploading artifacts...
WARNING: _build/: no matching files
ERROR: No files to upload
ERROR: Job failed: exit code 1