macOS: Fallback font list not ordered by writing system support

This can be reproduced by applying https://codereview.qt-project.org/#/c/215658/, and replicating the contents of the tst_QRawFont::unsupportedWritingSystem() test, but without removing the whitespace from the sample string.

After we added support for fallbacks for the common script to non-common multi engines, we will search for and find a fallback font for the space. Before we would search for it, but since we would only check the same engine over and over again, we would not find it. The consequence of this would be a performance hit at best, and at worst we could actually find some weird fallback font that contains a character for the whitespace.

The font merging mechanism is mainly intended as a mechanism to merge several fonts of a specific script. After the text of the common script started inheriting the preceding text's script, we also have to allow fallbacks in the list for this, and should trust the list provided by the font database to be in the right order and contain the right fonts. But we should not merge in fonts that provide glyphs for whitespace characters. The only real use case for displaying glyphs for whitespace characters is if the explicitly selected font supports them. Otherwise, there is no indication that the user would want these characters to be displayed.

edit: Changed title to describe the actual bug, as documented in the comments.