fix(tiff): The "tiff:half" hint was only applying to first MIP level (#5240)

IBA::make_texture() only passes the full metadata to the first level of
a MIPmap, and that might include output behavior hints, too! Make this
hint "sticky" in the TIFF writer, so if any subimage's first level's
spec enables half output, all the MIP levels of that subimage will allow
it.

Fixes #5239

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

src/tiff.imageio/tiffoutput.cpp

@@ -109,6 +109,7 @@ private:
     int m_outputchans;             // Number of channels for the output
     bool m_convert_rgb_to_cmyk;
     bool m_bigtiff;  // force bigtiff
+    bool m_allow_half = false;
 
     // Initialize private members to pre-opened state
     void init(void)
@@ -123,6 +124,7 @@ private:
         m_outputchans         = 0;
         m_convert_rgb_to_cmyk = false;
         m_bigtiff             = false;
+        m_allow_half          = false;
         ioproxy_clear();
     }
 
@@ -510,6 +512,13 @@ TIFFOutput::open(const std::string& name, const ImageSpec& userspec,
     const char* openmode = m_bigtiff ? (mode == AppendSubimage ? "a8" : "w8")
                                      : (mode == AppendSubimage ? "a" : "w");
 
+    if (m_spec.format == TypeDesc::HALF) {
+        // Look for hints about whether half output is allowed
+        m_allow_half
+            = m_spec.get_int_attribute("tiff:half",
+                                       OIIO::get_int_attribute("tiff:half"));
+    }
+
     // Open the file
 #if OIIO_TIFFLIB_VERSION >= 40500
     auto openopts = TIFFOpenOptionsAlloc();
@@ -625,8 +634,7 @@ TIFFOutput::open(const std::string& name, const ImageSpec& userspec,
         // unless the "tiff:half" attribute is nonzero -- use the global
         // OIIO attribute, but override with a specific attribute for this
         // file.
-        if (m_spec.get_int_attribute("tiff:half",
-                                     OIIO::get_int_attribute("tiff:half"))) {
+        if (m_allow_half) {
             m_bitspersample = 16;
         } else {
             // Silently change requests for unsupported 'half' to 'float'