Fix: Sun Beams shader does not work on Metal

Metal does not support the length function on integer vectors, so
explicitly cast to a float one.

source/blender/compositor/realtime_compositor/shaders/compositor_sun_beams.glsl

@@ -16,7 +16,7 @@ void main()
    * pixels along the integration path. Assume a minimum number of steps of 1 to avoid zero
    * division handling and return source pixels as is. */
   float integration_length = min(distance_to_source, max_ray_length);
-  float integration_length_in_pixels = length(input_size) * integration_length;
+  float integration_length_in_pixels = length(vec2(input_size)) * integration_length;
   int steps = max(1, int(integration_length_in_pixels));
   vec2 step_vector = (direction_to_source * integration_length) / steps;