BfPixelsWrapper: do not store the channel count and pixel type

The RGB channel count and pixel type/byte width can vary for every
series, so it is incorrect to store the values for the first series and
use them for every series.

Fixes ticket #10189.
BfPixelBuffer: allocate the correct size for the stack buffer

Should fix ticket #10202.
Merge pull request #673 from melissalinkert/buffer-sizes

BfPixelBuffer/BfPixelsWrapper buffer size fixes