When seting rectangle or ellipse selections to a fixed size, height or width in pixels, the actual width and/or height of the selections made have a random error of -1, 0 or +1 pixels.

Steps to reproduce:
1. Choose the rectangle or ellipse select tool.
2. Set the selection to 'fixed', "3x3".
3. Make a lot of selections in an image.

Expected results:
All selections are exactly 3x3 pixels in size.

Actual results:
The selections have sizes varying between 2x2, 2x3, 2x4, 3x2, 4x4 etc.

This looks like a rounding error. At any rate, it makes creating pixel-accurate test image patterns a pain.

This bug also occurs in GIMP 2.6.7 in OS X.