Fixed bug in SurfIsOffscreen.

This function is called by the frontend but it uses backend functionality to tesselate geometry.
It didn't clean up genereated tess data so backend received unnecessary,
geometry to render. It wasn't visible since viewport dimensions defaulted to zero values still
vulkan validation layer detected invalid vkCmdSetViewport parameters.

src/engine/renderer/tr_main.c

@@ -854,6 +854,7 @@ static qboolean SurfIsOffscreen( const drawSurf_t *drawSurf, vec4_t clipDest[128
 	// trivially reject
 	if ( pointAnd )
 	{
+        tess.numIndexes = 0;
 		return qtrue;
 	}
 
@@ -883,6 +884,7 @@ static qboolean SurfIsOffscreen( const drawSurf_t *drawSurf, vec4_t clipDest[128
 			numTriangles--;
 		}
 	}
+    tess.numIndexes = 0;
 	if ( !numTriangles )
 	{
 		return qtrue;