Improve arc drawing resolution

[laxsjo]

Fixed the bug where arc steps were rounded inconsistently.

I've also increased the default resolution when drawing arcs from 40 to 80.

Also added the new attribute resolution to the following functions to enable the user to manually specify a resolution:

• img-arc
• img-circle
• img-circle-sector
• img-circle-segment
• img-resolution

I also added the attribute to the documentation. The explanation might be a bit long though ;)

If anything is wrong or can be improved, just tell me.

[vedderb]

Looks good!