Add support for missing user ability to set Z_FIXED zlib strategy

CHANGELOG.md

@@ -8,6 +8,8 @@ For a complete change history, see the git log.
 
 ## Future
 
+- Added support for setting zlib `Z_FIXED` strategy with format string: `png:z=fixed`
+
 - Fixed handling of transparency level option in Octree-based PNG encoding (#1556)
 
 - Faster rendering of rasters by reducing memory allocation of temporary buffers (#1516)

src/image_util.cpp

@@ -198,6 +198,10 @@ void handle_png_options(std::string const& type,
                 {
                     *strategy = Z_RLE;
                 }
+                else if (s == "fixed")
+                {
+                    *strategy = Z_FIXED;
+                }
                 else
                 {
                     throw ImageWriterException("invalid compression strategy parameter: " + s);