Make touching-color test more robust against GPU imprecision

Previously, the `color-touching-tests.sb2` test "touches a color that
doesn't actually exist right now" would use a sprite with ghost 50,
blended against another sprite, to create the color that "doesn't
actually exist" when the query sprite is skipped. Unfortunately the
blend result was near a bit-boundary and, depending on the specific
hardware used, that test could fail on the GPU. When the renderer uses
the CPU path this test works fine, though, so the existing problem went
unnoticed.

To fix the problem I changed the project to use ghost 30 instead, which
results in a color that is less near a bit boundary and is therefore
less likely to fail on specific hardware.

As an example of what was happening: the `touching color` block was
checking for `RGB(127,101,216)` with a mask of `RGB(0xF8,0xF8,0xF0)`. On
the CPU it would find `RGB(120,99,215)`, which is in range, but on some
GPUs the closest color it could find was `RGB(119,98,215)` which
mismatches on all four of the least significant bits -- one of which is
enabled in the mask.

Author: Christopher Willis-Ford