SVG: Fix fts_01_0106.pdf

A gradient fill that doesn't fill the bbox should be see through
(unless background color is set, but we'll worry about that case
when I find an example file that uses it). Arrange for the pixmap
we draw the gradient fill into to be transparent initially.

Also ensure that when we convert to png we preserve transparency.
prevent potential heap access violation

fz_reset_gel fails to reset the length of active edges, which could
(AFAICT) lead to pointers in gel->active pointing to memory that's
been previously freed by fz_resize_array.
Update documentation of multithreading and contexts.

Attempt to improve the clarity of the explainations pertaining to
cloning contexts and bound contexts.
SVG: Fix clip stack handling etc.

fts_09_0919.pdf shows up some silly mistakes in the clip stack handling
and in the handling of 0 sized pixmaps. Simple fixes.
SVG: Add dumb group implementation.

Really just a structure indicator as SVG 1.1 doesn't support
blending etc.