Implement audio filters

Use Source:setFilter(type, params...) to set a filter.
Use Source:setFilter() to detach the current filter.
Use Source:getFilter() to get the currently set filter (or nil).

Squashed from pull request 27, and the clean-pr27 branch in love-experiments.

--HG--
branch : minor

src/modules/audio/null/Source.h

@@ -24,6 +24,7 @@
 // LOVE
 #include "common/Object.h"
 #include "audio/Source.h"
+#include "audio/Filter.h"
 
 namespace love
 {
@@ -79,6 +80,10 @@ public:
 	virtual int getFreeBufferCount() const;
 	virtual bool queue(void *data, size_t length, int dataSampleRate, int dataBitDepth, int dataChannels);
 
+	virtual bool setFilter(love::audio::Filter::Type type, std::vector<float> &params);
+	virtual bool setFilter();
+	virtual bool getFilter(love::audio::Filter::Type &type, std::vector<float> &params);
+
 private:
 
 	float pitch;