gpu: eliminate gaps by ensuring consistent transformations

This is another attempt at fixing the issue described in [0], the
previous attempt was reverted[1].

This change fixes the issue by tracking resolved transformations and
ensure that all segments within a path share a single transformation.

[0] https://github.com/linebender/piet-gpu/issues/62
[1] https://gioui.org/commit/2b21b48a7c5c4451deb642c164548a134bb9ad06

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/shaders/setup.h

@@ -35,4 +35,5 @@ struct Config {
     Alloc ptcl_alloc;
     Alloc pathseg_alloc;
     Alloc anno_alloc;
+    Alloc trans_alloc;
 };