The new QgsRenderContext::TextRenderFormat enum controls how text
should be handled during a render operation, e.g. whether to render
text as outlines (paths) or keep it as real text objects.
Deprecate previous arguments in QgsTextRenderer which handled
this same use case.
This allows us to make the setting vary per-render, instead of
having a single global flag controlling the setting. Ultimately
this will allow us to have different behaviour within the
canvas renders vs print layout exports.
Refs #3975