GRAPHICS: Fix dest alpha formula in blendPixelPtr

The original alpha computation formula had a problem:  If something was
drawn on top of a pixel that was already fully opaque, there would be
an overflow in the computed alpha, and the destination alpha would be
truncated to 0 (fully transparent).

In commit 264ba4a9 this formula was replaced with another one, which
did not have overflows but also was not correct.

This commits introduces a new formula, where the rounding errors have
been turned in another direction; drawing a fully opaque pixel on top
of a transparent one would result in a pixel which is almost, but not
fully, opaque.  However, this is no problem in practice, since drawing
fully opaque pixels can be achieved with much less code as a special
case, so add that (also improves rendering speed).
ANDROID: Set correct blending function

Overlays with alpha channel in ScummVM use pre-multiplied alpha.
Thus, is it necessary to set the source blend function to ONE, not
SOURCE_ALPHA.

While there is no firm decision on the texture format to be used to
cursors, make sure to set the key color to R=G=B=A=0.0, so that it
can be used with either pre-multiplied or non-pre-multiplied blending.