According to the Scintilla Documentation:

> Any markers not associated with a visible margin will be displayed as changes in background colour in the text. A width in pixels can be set for each margin. Margins with a zero width are ignored completely.

All lines with markers, either builtin or used by plugins (`git-changebar`), appear on the white background, which is ugly, especially in dark themes.

This MR disables rendering of markers by making them fully transparent on the overview.

Additionally, it fixes `SETMARGINWIDTHN` for the last (missing) margin:

> 5 margins are allocated initially numbered from 0 to SC_MAX_MARGIN (4). You can view, comment on, or merge this pull request online at:

https://github.com/geany/geany-plugins/pull/1147

-- Commit Summary --

* Overview: hide markers

-- File Changes --

M overview/overview/overviewscintilla.c (5)

-- Patch Links --

https://github.com/geany/geany-plugins/pull/1147.patch https://github.com/geany/geany-plugins/pull/1147.diff