Account for interleaved samples in Compute Layout and Allocation Size (#962)

The pixel format definitions use "sample" for a single component value
(an NV12 UV element is "composed of two Chroma samples", and an RGBA
pixel is four 8-bit samples). Compute Layout and Allocation Size used the
number of bytes per sample as the size of each position in a plane, which
undercounts planes that interleave several components: sourceLeftBytes
and sourceWidthBytes were half the correct value for the NV12 UV plane
and a quarter for RGBA, RGBX, BGRA and BGRX.

Multiply the bytes per sample by the number of samples stored at each
position of the plane.

Author: jyavenard

index.src.html

@@ -4303,7 +4303,10 @@
     7. While |planeIndex| < |numPlanes|:
         1. Let |plane| be the Plane identified by |planeIndex| as defined by
             |format|.
-        2. Let |sampleBytes| be the number of bytes per sample for |plane|.
+        2. Let |sampleBytes| be the number of bytes per sample for |plane|,
+            multiplied by the number of samples stored at each position of
+            |plane| (for example, 2 for the UV plane of {{NV12}} and 4 for
+            {{RGBA}}).
         3. Let |sampleWidth| be the horizontal [=sub-sampling factor=] of each
             subsample for |plane|.
         4. Let |sampleHeight| be the vertical [=sub-sampling factor=] of each