Define standard sample patterns (gpuweb#4054)

spec/index.bs

@@ -14237,15 +14237,29 @@ to the 2-dimensional rendering area of the <dfn dfn>framebuffer</dfn> -
 the set of render attachments in the current {{GPURenderPassEncoder}}.
 This rendering area is split into an even grid of pixels.
 
-Rasterization determines the set of pixels affected by a primitive. In case of multi-sampling,
-each pixel is further split into |descriptor|.{{GPURenderPipelineDescriptor/multisample}}.{{GPUMultisampleState/count}}
-samples. The locations of samples are the same for each pixel, but not defined in this spec.
-
-Issue: do we want to force-enable the "Standard sample locations" in Vulkan?
-
 The [=framebuffer=] coordinates start from the top-left corner of the render targets.
 Each unit corresponds exactly to a pixel. See [[#coordinate-systems]] for more information.
 
+Rasterization determines the set of pixels affected by a primitive. In case of multi-sampling,
+each pixel is further split into
+|descriptor|.{{GPURenderPipelineDescriptor/multisample}}.{{GPUMultisampleState/count}} samples.
+The <dfn dfn noexport>standard sample patterns</dfn> are as follows,
+with positions in framebuffer coordinates relative to the top-left corner of the pixel,
+such that the pixel ranges from (0, 0) to (1, 1):
+
+<table class=data>
+    <thead>
+        <tr><th>{{GPURenderPipelineDescriptor/multisample}}.{{GPUMultisampleState/count}}<th>Sample positions
+    <tbody>
+        <tr><td>1<td>
+            Sample 0: (0.5, 0.5)
+        <tr><td>4<td>
+            Sample 0: (0.375, 0.125)<br>
+            Sample 1: (0.875, 0.375)<br>
+            Sample 2: (0.125, 0.625)<br>
+            Sample 3: (0.625, 0.875)
+</table>
+
 Let's define a <dfn dfn>FragmentDestination</dfn> to contain:
 <dl dfn-for=FragmentDestination>
     : <dfn dfn>position</dfn>