Fix GrBufferAllocPool to include padding in bytes used for an allocation

[bsalomon, 2012-03-08 15:01:22]

When we allocate space in a vertex buffer we align the offset to be a multiple
of the vertex size. However, we weren't including the pad in the number of bytes
used. I also added additional validate calls to catch these types of bugs closer
to where they occur.

[TomH, 2012-03-08 15:04:06]

LGTM

[bsalomon, 2012-03-08 15:11:50]

Closed with 3339