fix(exr): handle edge case of exr attribute that interferes with our hints (#4008)

Our exr writer uses some special hints starting with "openexr:" to
control aspects of the writing, and the reader sets these to
communicate some things about the file it's reading.

But we identified a case where the actual exr file had metadata named
"openexr:levelmode" -- which is of no particular meaning to openexr
itself, but it is the name we use to communicate hints to our writer
by make_texture. We just need to make sure that if we don't set it
proactivesly, we make sure to clear it if there is a stray bit of
metadata with that same name coincidentally.

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/libOpenImageIO/maketexture.cpp

@@ -647,6 +647,8 @@ write_mipmap(ImageBufAlgo::MakeTextureMode mode, std::shared_ptr<ImageBuf>& img,
         if (!mipmap) {
             // Send hint to OpenEXR driver that we won't specify a MIPmap
             outspec.attribute("openexr:levelmode", 0 /* ONE_LEVEL */);
+        } else {
+            outspec.erase_attribute("openexr:levelmode");
         }
         // OpenEXR always uses border sampling for environment maps
         if (envlatlmode) {