v1.3.5-rc35 - Improved PNG file sizes for querystring commands

Four new querystring commands

• Add png.quality=0..100 which triggers quantization/lossy PNG compression.
• Add png.min_quality=0..100 which determines when to fallback to lossless PNG.
• Add png.quantization_speed=1..11 (default 4) to determine how much time to spend compressing
• Add png.libpng=true to switch back to libpng encoder (which produces larger files)