Round up sub-sampled width in Compute Layout and Allocation Size (#964)

Step 9 computed sourceWidthBytes using the integer division of the rect
width by the horizontal sub-sampling factor, while step 7 rounds the
sub-sampled height up and the pixel format definitions round both
dimensions up. For a rect with an odd width, this dropped the last chroma
column: a 3x3 I420 frame got a chroma stride of 1 and an allocation size
of 13 instead of 2 and 17, as expected by
webcodecs/videoFrame-odd-size.any.js.

Round the division up, matching step 7.

Fixes #963

Author: jyavenard

index.src.html

@@ -4324,9 +4324,9 @@
             truncated |parsedRect|.{{DOMRectInit/x}} by |sampleWidth|,
             multiplied by |sampleBytes|.
         9. Set |computedLayout|'s [=computed plane layout/sourceWidthBytes=] to
-            the result of the integer division of
-            truncated |parsedRect|.{{DOMRectInit/width}} by |sampleWidth|,
-            multiplied by |sampleBytes|.
+            the result of the division of truncated
+            |parsedRect|.{{DOMRectInit/width}} by |sampleWidth|, rounded up to
+            the nearest integer, multiplied by |sampleBytes|.
         10. If |layout| is not `undefined`:
             1. Let |planeLayout| be the {{PlaneLayout}} in |layout| at position
                 |planeIndex|.