mesa: Round the argument to PixelStoref instead of truncating.

mesa: Round the argument to PixelStoref instead of truncating.

From the GL 2.1 specification, page 114 (page 128 of the PDF):

    "The version of PixelStore that takes a floating-point value
     may be used to set any type of parameter; if the parameter is
     boolean, then it is set to FALSE if the passed value is 0.0
     and TRUE otherwise, while if the parameter is an integer, then
     the passed value is rounded to the nearest integer."

Fixes piglit roundmode-pixelstore.
Note: This is a candidate for the 7.11 branch.

Reviewed-by: Brian Paul <brianp@vmware.com>