Fix a bunch of signed overflow issues

Fix a bunch of signed overflow issues

In pixman-fast-path.c: (1 << 31) - 1 causes a signed overflow, so
change to (1U << n) - 1.

In pixman-image.c: The check for whether m10 == -m01 will overflow
when -m01 == INT_MIN. Instead just check whether the variables are 1
and -1.

In pixman-utils.c: When the depth of the topmost channel is 0, we can
end up shifting by 32.

In blitters-test.c: Replicating the mask would end up shifting more
than 32.

In region-contains-test.c: Computing the average of two large integers
could overflow. Instead add half the difference between them to the
first integer.

In stress-test.c: Masking the value in fake_reader() would sometimes
shift by 32. Instead just use the most significant bits instead of
the least significant.

All these issues were found by the IOC tool:

    http://embed.cs.utah.edu/ioc/