Expose "missingtexture" and "missingalpha" to all the texture functions.

These were supported in the underlying OIIO texture options, but there was no way to set them from OSL.

The idea is that by passing a "missingtexture" (and optionally "missingalpha") parameter to texture(), texture3d(), or environment(), it instructs the texture system to substitute that color for textures that are missing or broken, and NOT treat them as an error.  If this optional argument is not present, a missing or broken texture will be treated as an error, as usual.