Icon-O-Matic: Add perspective transformations

As part of adding perspective transformations, agg_trans_perspective.h
was patched to fix a multiple definitions error. This change has been
submitted for review to the "upstream" repositories at [1], [2],
and [3].

Also includes various other improvements such as VertexSource being
split into its own file, code style improvements, and documentation
improvements.

[1] https://sourceforge.net/p/agg/patches/6/
[2] https://github.com/ghaerr/agg-2.6/pull/9
[3] https://github.com/aggeom/agg-2.6/pull/7

Change-Id: I4bffd2f87354bde10155e23145a232a925be6ff3
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6801
Reviewed-by: Adrien Destugues <pulkomandy@pulkomandy.tk>