Nested [size]/[font]/[color] tags are not converted correctly

Nested [size]/[font]/[color] tags are not converted correctly. This issue was reported also here.
I contribute a patch for this issue. Please review and apply it.

example code is as follows;

[size=10]10px [size=20]20px [size=30]30px [/size]20px [/size]10px[/size]

[size=20][color=blue]BLUE [color=red]RED [/color][color=green]GREEN [/color]BLUE[/color] [/size]

[size=20][font=Arial]Arial [font=Century]Century [/font][font=Comic sans MS]Comic sans MS [/font]Arial [font=Arial Bold Italic]Arial Bold Italic [/font]Arial [/font][/size]