gpu: Handle tiny offscreens

Due to rounding errors, it is possible after intersecting a lot of
rectangles to end up with a tiny size for an offscreen. And because we
allow an epsilon before ceil()ing to an integer (see commit afc7b46264
for details) it is now possible that we end up with a size of 0.

Avoid that by always enforcing a minimum size of 1px.

Test included

The test uses a different codepath to arrive at the same problem - it
specifies the small size instead of triggering it via rounding errors
and clipping like the original bug (and most likely the more common case
to encounter this problem.

Fixes #6656

gsk/gpu/gskgpunodeprocessor.c

@@ -345,8 +345,8 @@ gsk_gpu_node_processor_init_draw (GskGpuNodeProcessor   *self,
 
   area.x = 0;
   area.y = 0;
-  area.width = ceilf (graphene_vec2_get_x (scale) * viewport->size.width - EPSILON);
-  area.height = ceilf (graphene_vec2_get_y (scale) * viewport->size.height - EPSILON);
+  area.width = MAX (1, ceilf (graphene_vec2_get_x (scale) * viewport->size.width - EPSILON));
+  area.height = MAX (1, ceilf (graphene_vec2_get_y (scale) * viewport->size.height - EPSILON));
 
   image = gsk_gpu_device_create_offscreen_image (gsk_gpu_frame_get_device (frame),
                                                  FALSE,

testsuite/gsk/compare/repeat-tiny-nogl-nocairo.node

@@ -0,0 +1,13 @@
+color {
+  color: black;
+  bounds: 0 0 20 20;
+}
+
+repeat {
+  bounds: 0 0 20 20;
+  child: color {
+    color: red;
+    /* 1/1024 */
+    bounds: 0 0 0.0009765625 0.0009765625;
+  }
+}

testsuite/gsk/meson.build

@@ -136,6 +136,7 @@ compare_render_tests = [
   'repeat-repeats-nested-nogl',
   'repeat-scaling',
   'repeat-texture',
+  'repeat-tiny-nogl-nocairo',
   'repeating-gradient-scaled',
   'reuse-of-texture-nested-in-offscreens',
   'rounded-clip-with-huge-bounds-nogl',