Fix bugs in \be output

* The 11rcombs/libass@7505c18 merge broke the top two pixel rows in be_blur_c.
* The assembler versions of be_blur were missing the y=2 loop initializer, so a jarring extra pixel row was output at the top.
* The assembler versions of be_blur didn’t check x<w before going into the non-vectorized trailing loop, so an extra pixel column was output sometimes (depending on the width).