Revert "Change change.c to treat text scale as mils" and fix harder..

Revert "Change change.c to treat text scale as mils" and fix harder..

This reverts commit 9396377717f7c49cf38d753a35e32d004294b09e and
fixes what was probably the underlying issue... I made a mistake in
commit 2d21a8ad8f38e07b17450450a096f1ce4b3bef0e such that it added
the existing text scale when setting an "absolute" text size, but
ignored it when setting a relative change. (Clearly backwards).

Additionally, I've switched to computing the new value in double
arithmetic to avoid a potential overflow when the input value is
large and we are using a 32-bit Coord type.

See commit 2d21a8ad8f38e07b17450450a096f1ce4b3bef0e for a description
of how text scale is defined and used. If we want to change how our
text is sized / scaled in the future, we would need to change usage
in many more places than the reverted commit touched.