Implement ffmpeg-independent PNGWriter (closes #476)

Creates a dedicated folder for each video and does not depend on ffmpeg, just
like the EXRWriter (but supporting all MC versions).

Also does 32 bit depth export by chunking the 32-bit float into the 8-bit BGRA
component channels.

src/main/java/com/replaymod/render/RenderSettings.java

@@ -72,7 +72,7 @@ public class RenderSettings {
 
         EXR(null, "exr"),
 
-        PNG("\"%FILENAME%-%06d.png\"", "png");
+        PNG(null, "png");
 
         private final String preset;
         private final String fileExtension;