render.Clear sets alpha incorrectly for upcoming surface calls in a rendertarget

for example,

```
render.SetRenderTarget(some_rt)
  render.Clear(0,0,0,100)
  surface.SetDrawColor(255,255,255,200)
  surface.DrawRect(0,0,100,100)
```

this will draw the rectangle with color
255,255,255,(200/255)*100

i am assuming that it should however set the color of the whole rendertarget at black with 100 alpha
then let the surface calls that are upcoming do whatever color they want
