Align OSL image alpha with texture conventions#2985
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This changelist fixes the alpha output of the OSL `image` and `hextiledimage` nodes when sampling a file with fewer than four channels. The OSL `texture` function returns zero for channels not present in the file, so a `color4` or `vector4` `image` reading an RGB file returned a transparent rather than opaque result. The fix queries the channel count with `gettextureinfo` and sets the output alpha to one when fewer than four channels are present, matching the GLSL and MSL backends. More broadly, this change aligns the `image` node with the established convention that a missing fourth channel resolves to one, as already followed by the MaterialX `convert` node, the OpenUSD `UsdUVTexture` node, and standard GPU texture sampling. The specification text for the `image` node, previously ambiguous on this point, has been clarified to note the fourth-channel exception.
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From our automated render test suite, here are before and after comparisons of the color4 Blur Render Comparison (Before): 
Blur Render Comparison (After): 
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This changelist fixes the alpha output of the OSL
imageandhextiledimagenodes when sampling a file with fewer than four channels. The OSLtexturefunction returns zero for channels not present in the file, so acolor4orvector4imagereading an RGB file returned a transparent rather than opaque result. The fix queries the channel count withgettextureinfoand sets the output alpha to one when fewer than four channels are present, matching the GLSL and MSL backends.More broadly, this change aligns the
imagenode with the established convention that a missing fourth channel resolves to one, as already followed by the MaterialXconvertnode, the OpenUSDUsdUVTexturenode, and standard GPU texture sampling. The specification text for theimagenode, previously ambiguous on this point, has been clarified to note the fourth-channel exception.