Fix compression quality for image file formats like jpeg. (#50)

This fixes an issue introduced by #31
that causes the compression quality to be ignored for formats with
default compression like JPEG files. In the case of these files,
the compression string was not being set and so the line that was
appending the quality was actually triggering behavior that caused
the quality value to be ignored. This in turn started causing the
Natron-Tests to start failing because the JPEG writer used in these
tests was not honoring the requested quality value.

The fix here is simply to specify a valid base compression string
for these formats so that when the quality is appended, the
compression/codec part of the string is not empty. This avoids
triggering logic in OIIO that ignores the quality values for
empty compression/codec strings.

OIIO/WriteOIIO.cpp

@@ -1015,8 +1015,16 @@ WriteOIIOPlugin::beginEncodeParts(void* user_data,
     string compression;
 
     switch ((EParamCompression)compression_i) {
-    case eParamCompressionAuto:
-        break;
+    case eParamCompressionAuto: {
+        // Set compression string for formats that only have a single compression type.
+        static const char* formats[] = {"jpeg", "webp", "heic", "avif", nullptr};
+        for (int i = 0; formats[i] != nullptr; ++i) {
+            if (strcmp(data->output->format_name(), formats[i]) == 0) {
+                compression = formats[i];
+                break;
+            }
+        }
+    } break;
     case eParamCompressionNone: // EXR, TIFF, IFF
         compression = "none";
         break;