Incorrect code in qgrayraster.c causing slow rendering

Hi,

I recently used a profilier to check why my rendering was so slow ahen i was drawing a path.

I tracked down the function which was taking up all the time and it was
static void gray_render_cubic( RAS_ARG_ const QT_FT_Vector* control1,
const QT_FT_Vector* control2,
const QT_FT_Vector* to )

This code appears to be wrong in a few cases.
1: In the first instance there seems to be a typo in the line:
dy = DOWNSCALE( ras.y ) + to->y - 3 * ( control1->x + control2->y );
Should this not be control1->y + control2->y
2: The value of dx appears to be huge on occasion for no apparent reason.

Hope this helps