* fix(render): check the nesting limit for mask content and bound GIF frame buffers
A mask's content is drawn straight from mask_pixels, not through
draw_layer, so masks nested inside masks recursed past the 256-level limit
and down the stack. GIF decoding budgeted frames by the logical canvas but
left the decoder's allocation limits off, so one frame descriptor could ask
for a gigabyte before the frame came back to be checked.
* address review: give the GIF decoder one frame of working room beyond the animation cap