Align OSL image alpha with texture conventions

[jstone-lucasfilm]

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.

[jstone-lucasfilm]

From our automated render test suite, here are before and after comparisons of the color4 blur node, providing some visual intuition for the improved alignment across languages:

Blur Render Comparison (Before):

Blur Render Comparison (After):

[meshula]

This looks good to me, populating an un-supplied alpha with one will make users happy and matches the convention we use in Hydra!