Introduce paintSolidColorImageMask command to handle 1x1 solid image 

Fixes #4436
This line causes rendering problems on Firefox (v. 32, Ubuntu 14.04):

![screenshot from 2014-10-06 14 21 14](https://cloud.githubusercontent.com/assets/822035/4525420/87d0b2f0-4d53-11e4-9450-50843dda2481.png)

If I change it to:
```javascript
this.ctx.fillStyle = 'transparent';
this.ctx.fillRect(0, 0, 1, 1);
```
the problem with the black boxes is solved, but that would defeat the purpose of this commit. Any suggestions here?