Overhaul gfx command target/source image parameter parsing/handling

(closes #715)

This cleans up parser-quirk-gfx's target/source image parsing and
the associated AST build up there and in rtl-gfx.
Clean up various rtl-gfx definitions to use proper default args
parser: Remove the now unused FB_VAREXPROPT_ALLOWADDROF
parser: Remove unnecessary fbSetIsExpression() around cExpression()

(cExpression() already does that)