Fix opengl MULTIPLY blend mode

This changes the opengl multiply blend mode implementation so the output matches the default renderer.

Consider:
```java
void setup() {
  size(500,500);
  background(255);
  noStroke();
}

void draw() {
  background(255);
  blendMode(BLEND);
  fill(#3D59AB);
  ellipse(width*0.3, height*0.5, width*0.5, height*0.5);
  blendMode(MULTIPLY);
  fill(#FFD700);
  ellipse(width*0.7, height*0.5, width*0.5, height*0.5);
}
```
Result:
![output](https://cloud.githubusercontent.com/assets/1057304/3308283/6361653e-f67a-11e3-95f8-089ceeeb8f10.png)

Change the size method to use P2D instead and you get:
![p2d_output](https://cloud.githubusercontent.com/assets/1057304/3308298/bcaa917e-f67a-11e3-9e2e-dfc02757d4f6.png)

This change makes the P2D renderer output the same as the JAVA2D renderer.