oleaut32: In variants check bOverflow to make sure not to accidently overun static buffers in an overflow case (Coverity 313).

The simplification was bad, native msxml works if interface pointers
to the wrong interface are passed in. Tests included.

This should fix the heap corruption that caused intermittent crashes
for patchwatcher.

caps.MaxTextureWidth / caps.MaxTextureHeight can potentially be very
large, which could cause a driver to run out of memory. Arguably the
driver shouldn't report such large limits, but there's no reason for
us to create a texture larger than 1024x1024 either.