Fix parsing RGBA string in wxColour::FromString()

Using "%20c" in scanf() doesn't work unless we really have exactly 20
characters, so use "%20[^)]" to take up to 20 characters until the closing
parenthesis instead.

Closes #17739.

docs/changes.txt

@@ -117,6 +117,7 @@ All (GUI):
 - Fix position of the rectangle returned by wxDataViewCtrl::GetItemRect().
 - Add wxDataViewRenderer::GetAccessibleDescription().
 - Improve wxImage::Scale() handling of pixels with alpha channel (Tim Kosse).
+- Fix parsing of RGBA strings in wxColour (Laurent Poujoulat).
 
 wxGTK: