This makes the code more closely match mpv's style. Specifically, it
changes some names from camelCase to snake_case, removes some Hungarian
notation, replaces direct vtbl access with COM macros, makes use of the
SAFE_RELEASE macro, moves some declarations closer to their first use,
and fixes the brace style, as well as a few other things.
This also makes the IDropTargetVtbl static and it fixes the buggy
QueryInterface implementation.
This was mostly self-contained, so its removal makes w32_common.c a bit
easier to read. Also, because it was self contained and its author has
agreed to LGPL relicencing, the new file has the LGPL licence header.
