UPBGE: Restore bind id for VSM shadow.

Previously the shadow bind id returned by shadowBindId was the depth
map in case of simple shadow or the "moment" shadow in case of VSM.
This was recently unified to return only the depth map in the last
shadow refactor but broken some files using custom shader with VSM
"moment" map.
The previous behaviour is restored while waiting for a better API to
access to the two map in case of VSM.

Fix issue #576.

source/blender/gpu/intern/gpu_material.c

@@ -2873,7 +2873,13 @@ int GPU_lamp_shadow_buffer_type(GPULamp *lamp)
 
 int GPU_lamp_shadow_bind_code(GPULamp *lamp)
 {
-	return lamp->depthtex ? GPU_texture_opengl_bindcode(lamp->depthtex) : -1;
+	if (lamp->tex) {
+		return GPU_texture_opengl_bindcode(lamp->tex);
+	}
+	else if (lamp->depthtex) {
+		return GPU_texture_opengl_bindcode(lamp->depthtex);
+	}
+	return -1;
 }
 
 const float *GPU_lamp_dynpersmat(GPULamp *lamp)