Set lightGrid point colour to r_forceAmbient after overbright shift

Avoid overflow and incorrect results due to the overbright shift.

Also fix `r_forceAmbient` affecting lightGrid points inside of walls.

src/engine/renderer/tr_bsp.cpp

@@ -4124,13 +4124,6 @@ void R_LoadLightGrid( lump_t *l )
 		tmpDirected[ 2 ] = in->directed[ 2 ];
 		tmpDirected[ 3 ] = 255;
 
-		const byte forceAmbientNormalised = floatToUnorm8( r_forceAmbient.Get() );
-		if ( tmpAmbient[0] < forceAmbientNormalised &&
-			tmpAmbient[1] < forceAmbientNormalised &&
-			tmpAmbient[2] < forceAmbientNormalised ) {
-			VectorSet( tmpAmbient, forceAmbientNormalised, forceAmbientNormalised, forceAmbientNormalised );
-		}
-
 		if ( tr.legacyOverBrightClamping )
 		{
 			R_ColorShiftLightingBytes( tmpAmbient );
@@ -4143,6 +4136,18 @@ void R_LoadLightGrid( lump_t *l )
 			directedColor[ j ] = tmpDirected[ j ] * ( 1.0f / 255.0f );
 		}
 
+		const float forceAmbient = r_forceAmbient.Get();
+		if ( ambientColor[0] < forceAmbient &&
+			ambientColor[1] < forceAmbient &&
+			ambientColor[2] < forceAmbient &&
+			/* Make sure we don't change the (0, 0, 0) points because those are points in walls,
+			which we'll fill up by interpolating nearby points later */
+			ambientColor[0] != 0 &&
+			ambientColor[1] != 0 && 
+			ambientColor[2] != 0 ) {
+			VectorSet( ambientColor, forceAmbient, forceAmbient, forceAmbient );
+		}
+
 		// standard spherical coordinates to cartesian coordinates conversion
 
 		// decode X as cos( lat ) * sin( long )