Added a non-zero test for one of the diagonal values.

It now recognizes scaled variants of the identity matrix, too.

Get rid of almost all uses of these definitions.  They're still defined for
delinquent out-of-tree drivers, and also for the Mesa build.  As well as
for miinitext.c.  But largely gone.

Now, we only check for filter commonality if we're operating on a source
picture, and we compare the id (screen-independent index of the filter name)
rather than the pointer to the filter (per-screen state).

Now, filters may only be set on source pictures when the filter is common to
all screens.  Also, like SetPictureTransform, ChangePictureFilter is now not
called on source pictures.

A screen's ChangePictureTransform now isn't called when changing the transform,
as source pictures aren't associated with screens.  Also, attempting to set
an AlphaMap to a source picture will fail with BadMatch just like a Window
would, preventing another crash.

Instead, stick the NULL return default case afterwards, so that the compiler can
warn us when we've got unimplemented cases.  Removes some unimplemented and
unused 8bpp, depth 4 picture format names.