MRF: Adjust PNG limits (#5347)

Don't allow uncompressed PNGs to be generated.
Increase the output buffer size to avoid overflow.

frmts/mrf/PNG_band.cpp

@@ -237,8 +237,11 @@ CPLErr PNG_Codec::CompressPNG(buf_mgr &dst, buf_mgr &src)
 #endif
 
     // Let the quality control the compression level
-    // Supposedly low numbers work well too while being fast
-    png_set_compression_level(pngp, img.quality / 10);
+    // Start at level 1, level 0 means uncompressed
+    int zlvl = img.quality / 10;
+    if (0 == zlvl)
+        zlvl = 1;
+    png_set_compression_level(pngp, zlvl);
 
     // Custom strategy for zlib, set using the band option Z_STRATEGY
     if (deflate_flags & ZFLAG_SMASK)
@@ -359,7 +362,8 @@ PNG_Band::PNG_Band( MRFDataset *pDS, const ILImage &image,
         return;
     }
     // PNGs can be larger than the source, especially for small page size
-    poDS->SetPBufferSize( image.pageSizeBytes + 100);
+    // If PPNG is used, the palette can take up to 2100 bytes
+    poDS->SetPBufferSize(static_cast<unsigned int>(1.1 * image.pageSizeBytes + 4000));
 }
 
 NAMESPACE_MRF_END