Previously to support dynamic reconfigurations of the matrix string (e.g. 0m),
the rdiv values would always be cleared to 0.f, causing the rdiv to be
recalculated based on the new filter. This however had the side effect of
always ignoring user specified rdiv values.

Instead float user_rdiv[0] is added to ConvolutionContext which will store the
user specified rdiv values. Then the original rdiv array will store either the
user_rdiv or the automatically calculated 1/sum.

This fixes trac ticket #10294, #10867.
Signed-off-by: Stone Chen <chen.stonechen@gmail.com>
Signed-off-by: Marton Balint <cus@passwd.hu>