til: fixup til_fb_fragment_t.texture fragmenting

Until now when fragmenting with a texture present the texture
pointer was simply copied through to the new logical fragment.
The problem with that is when sampling pixels from the texture in
a nested frame scenario, the locations didn't align with the
placement of the logical fragment.

With this change when the incoming fragment has a texture, the
output fragment gets some uninitialized memory attached in the
outgoing fragment's texture pointer.  Then the fragmenter is
expected to do the same populating of res_fragment->texture it
already did for res_fragment, just relative to
fragment->texture->{buf,stride,pitch} etc.

It's a bit hairy/janky because til_fb_fragment_t.texture is just
a pointer to another til_fb_fragment_t.  So the ephemeral/logical
fragments fragmenting/tiling produces which tend to just be
sitting on the stack need to get another til_fb_fragment_t
instance somewhere and made available at the ephemeral
til_fb_fragment_t's .texture member.  We don't want to be
allocating and freeing these things constantly, so for now I'm
just ad-hoc stowing the pointer of an adjacent on-stack texture
fragment in the .texture member when the incoming fragment has a
texture.  But this is gross because the rest of the fragment
contents don't get initialized _at_all_, and currently if the
incoming fragment has no texture the res_fragment->texture member
isn't even initialized.  The fragmenters aren't really supposed
to be expecting anything sensible in *res_fragment, but now we're
making use of res_fragment->texture *if* fragment->texture is
set.  This is just gross.

So there's a bunch of asserts sprinkled around to help police
this fragility for now, but if someone writes new fragmenters
there's a good chance this will trip them up.

1 files changed, 4 insertions, 1 deletions

diff --git a/src/til.c b/src/til.c
index 1bc03b4..151f690 100644
--- a/src/til.c
+++ b/src/til.c
@@ -189,7 +189,10 @@ static void module_render_fragment(til_module_context_t *context, til_threads_t
 			til_threads_wait_idle(threads);
 		} else {
 			unsigned		fragnum = 0;
-			til_fb_fragment_t	frag, *frag_ptr = &frag;
+			til_fb_fragment_t	frag, texture, *frag_ptr = &frag;
+
+			if ((*fragment_ptr)->texture)
+				frag.texture = &texture; /* fragmenter needs the space */
 
 			while (frame_plan.fragmenter(context, *fragment_ptr, fragnum++, &frag))
 				module->render_fragment(context, ticks, 0, &frag_ptr);