Fix excessive memory usage when saving EXR file with many channels (A…

…cademySoftwareFoundation#3176)

Scratch memory allocation in ImageOutput::to_native_rectangle was using
the number of channels squared.

src/libOpenImageIO/imageoutput.cpp

@@ -364,7 +364,7 @@ ImageOutput::to_native_rectangle(int xbegin, int xend, int ybegin, int yend,
     // (cases #3 and #4 above).
     imagesize_t contiguoussize = contiguous
                                      ? 0
-                                     : rectangle_values * input_pixel_bytes;
+                                     : rectangle_pixels * input_pixel_bytes;
     contiguoussize             = (contiguoussize + 3)
                      & (~3);  // Round up to 4-byte boundary
     OIIO_DASSERT((contiguoussize & 3) == 0);