Padding overlaps when line numbers are disabled

Merged iain requested to merge iainx/gnome-builder:dev/iain/fix-1908 into main

If all gutter adornments are disabled (line numbers, diagnostics & line changes) there is an extra 2px padding drawn in the gutter background colour

When calculating the size set it to zero if none of the gutters adornments are present

Fixes #1908 (closed)

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