Add Buffer:clear, to reset all or part of a Buffer's contents to 0.

src/modules/graphics/Buffer.h

@@ -131,6 +131,11 @@ public:
 	 */
 	virtual bool fill(size_t offset, size_t size, const void *data) = 0;
 
+	/**
+	 * Reset the given portion of this buffer's data to 0.
+	 */
+	virtual void clear(size_t offset, size_t size) = 0;
+
 	/**
 	 * Copy a portion of this Buffer's data to another buffer, using the GPU.
 	 **/