Remove virtual readPixel() function from RenderDevice

This function is only used by SDLSoftwareRenderDevice, so it doesn't
need to be declared for all render devices.
Remove dstrect parameter from fillWithColor()

All of our uses for this function were passing NULL to this variable in
order to fill the entire image surface.
Always show quantity in tooltips when quantity > 1

Sometimes, it's difficult to read the quantity that's displayed on the
icon, especially for quantities > 100. This provides some easier to read
text in the item tooltip.
Merge pull request #1224 from dorkster/renderer

Remove virtual readPixel() function from RenderDevice
Merge pull request #1226 from dorkster/item_tooltips

Always show quantity in tooltips when quantity > 1