Implement 24 bit support for WAV export (#3021)

Add a new value of "24 Bit Float" to the "Depth" combo box in the
project export dialog.

Add a new enum value to ProjectRenderer::Depth and extend the evaluation
of the different enum values in ProjectRenderer.

Add the new case of a depth of 24 to AudioFileWave and remove some
repetition with regards to SF_FORMAT_WAV in the code. It's only set once
now and then the depth is "added" in a switch statement.

include/ProjectRenderer.h

@@ -44,6 +44,7 @@ public:
 	enum Depths
 	{
 		Depth_16Bit,
+		Depth_24Bit,
 		Depth_32Bit,
 		NumDepths
 	} ;