Optimize StreamBuffer::getUsableSize to not be a virtual method.

src/modules/graphics/StreamBuffer.h

@@ -54,8 +54,8 @@ public:
 
 	size_t getSize() const { return bufferSize; }
 	BufferType getMode() const { return mode; }
+	size_t getUsableSize() const { return bufferSize - frameGPUReadOffset; }
 
-	virtual size_t getUsableSize() const = 0;
 	virtual MapInfo map(size_t minsize) = 0;
 	virtual size_t unmap(size_t usedsize) = 0;
 	virtual void markUsed(size_t usedsize) = 0;
@@ -67,6 +67,7 @@ protected:
 	StreamBuffer(BufferType mode, size_t size);
 
 	size_t bufferSize;
+	size_t frameGPUReadOffset;
 	BufferType mode;
 
 }; // StreamBuffer