Arrows in sort-2x.gif are too big and overlap

[TobiasBg]

In the HiDPI version of the sort arrows (e.g. in the head row of WP_List_Tables), the individual arrows are too big and extend beyond the center of the image.

The attached screenshot (zoomed in on iPad 3) shows that the lower right corner is missing, while the corner of the down arrow is already showing in the top right corner.

[TobiasBg]

Arrows in sort-2x.gif extend beyond horizontal center of the image

[lessbloat]

sort-2x.gif​ should fix this.

[lessbloat]

sort-2x-alt.gif​ as an alternative removes the double solid baseline for each arrow. Here's a comparison of the two:

[helen]

sort-2x-alt.gif​ looks nice to me.

[nacin]

In 23245:

Fix the sprite positioning for HiDPI sorting arrows.

props lessbloat.
see #23071.
for trunk.

[nacin]

Debating whether to bother with a CSS version bump for the 3.5 branch.

[ryan]

In 23287:

Fix the sprite positioning for HiDPI sorting arrows.

props lessbloat.
fixes #23071.
for 3.5.