Nyall Dawson 4022c5fc38 Make QgsSymbolLayerUtils::symbolPreview* methods const, and utilise
internal symbol clones

Rendering symbols is a NON-CONST operation, which can permanently
alter the symbol instance (e.g. via changes made by the symbol
or symbol layer's startRender methods.).

This makes debugging super complex - because methods which look
like they are just generating previews of symbols can actually
change the original symbol instances, resulting in permanent changes
to a layer's style.

Refs #19910 (specifically, me pulling my hair out trying to deduce
seemingly random changes to layer's symbols)
2019-01-25 23:27:52 +11:00
..