Fix readTextureToTexelViews for cubemaps

[greggman]

The issue was that the code that reads back the texture uses textureGather for cubemaps. It has to use textureGather because it's the only way to load from cubemaps with integer formats. It was computing a texel coord and then doing the common conversion to normalized texture coord

normalizedCoord = (integerTexelCoord + 0.5) / dimension

But, for textureGather, 0.5 is the center of the texel and so it's an edge case. The gather could go in any direction from the center. Moving it off center should fix that issue.

Given coord C in the center of a texel

+-------+-------+-------+
|       |       |       |
|       |       |       |
|       |       |       |
+-------+-------+-------+
|       |       |       |
|       |   C   |       |
|       |       |       |
+-------+-------+-------+
|       |       |       |
|       |       |       |
|       |       |       |
+-------+-------+-------+

textureGather could return any of these 4 results

+-------+-------+-------+      +-------+-------+-------+
|.......|.......|       |      |       |.......|.......|
|.......|.......|       |      |       |.......|.......|
|.......|.......|       |      |       |.......|.......|
+-------+-------+-------+      +-------+-------+-------+
|.......|.......|       |      |       |.......|.......|
|.......|...C...|       |      |       |...C...|.......|
|.......|.......|       |      |       |.......|.......|
+-------+-------+-------+      +-------+-------+-------+
|       |       |       |      |       |       |       |
|       |       |       |      |       |       |       |
|       |       |       |      |       |       |       |
+-------+-------+-------+      +-------+-------+-------+




+-------+-------+-------+      +-------+-------+-------+
|       |       |       |      |       |       |       |
|       |       |       |      |       |       |       |
|       |       |       |      |       |       |       |
+-------+-------+-------+      +-------+-------+-------+
|.......|.......|       |      |       |.......|.......|
|.......|...C...|       |      |       |...C...|.......|
|.......|.......|       |      |       |.......|.......|
+-------+-------+-------+      +-------+-------+-------+
|.......|.......|       |      |       |.......|.......|
|.......|.......|       |      |       |.......|.......|
|.......|.......|       |      |       |.......|.......|
+-------+-------+-------+      +-------+-------+-------+

if we move it off center

+-------+-------+-------+
|       |       |       |
|       |       |       |
|       |       |       |
+-------+-------+-------+
|       |       |       |
|       |       |       |
|       |     C |       |
+-------+-------+-------+
|       |       |       |
|       |       |       |
|       |       |       |
+-------+-------+-------+

Then it will always return the same result

+-------+-------+-------+
|       |       |       |
|       |       |       |
|       |       |       |
+-------+-------+-------+
|       |.......|.......|
|       |.......|.......|
|       |.....c.|.......|
+-------+-------+-------+
|       |.......|.......|
|       |.......|.......|
|       |.......|.......|
+-------+-------+-------+

[shrekshao]

LGTM