d3d: Fix CheckAlpha() for 16-bit formats.

Not reversed in d3d, unlike gles.  Fixes #6883.
d3d: Fix some missing Releases() in debug code.
Merge pull request #6886 from unknownbrackets/d3d9

d3d: Fix CheckAlpha() for 16-bit formats
d3d: Oops, fix dumb CheckAlpha() mistake.