[REGRESSION] Fix crash when using transparency (#453)

Bug was introduced in 188068f
Enabling transparency with setTransparent( 0.5f ) would cause a crash if
point light shadow mapping is enabled.

A new shader code snippet should only be included in shadow casters if
doing alpha hashing.

Fixes #453

Samples/Media/Hlms/Pbs/Any/Main/800.PixelShader_piece_ps.any

@@ -1008,7 +1008,9 @@
 		@insertpiece( SampleDiffuseMap )
 	@end
 	@insertpiece( DoShadowCastPS )
-	@insertpiece( ProcessAlpha )
+	@property( hlms_alpha_hash )
+		@insertpiece( ProcessAlpha )
+	@end
 @end
 
 @end