#5 Overflow while alpha blitting

[Dmitry Yakimov]

Hi,

1.
File agg_pixfmt_rgba.h, line 1739 should be:

Blender::blend_pix(p, cr, cg, cb, alpha, alpha);

2.
File agg_pixfmt_rgba.h, line 178 should be:

cover <<= (base_shift - 8);

(we should not add +1 there or we get overflow when cover == 255). btw, what this line is about?