Various Windows and MSVC fixes
silk: Fix incorrect ifdef in debug.c
celt: Fix broken SSE pre-processor check due to typo This broke 5 years ago in 43120f00758219a784f952754f33b9788a8d731b
Replace WIN32 with _WIN32 everywhere _WIN32 is defined on all Windows platforms by every compiler that targets Windows. We do not need WIN32 at all.
repacketizer_demo: check for read errors to fix compiler warnings Actually check for read errors instead of just storing the return value in a variable that then never gets checked. Also fixes "conversion from 'size_t' to 'int', possible loss of data" compiler warnings on Windows with MSVC caused by storing the size_t returned by fread() into an int variable.
Fix MSVC warning about trunction from double to float Specify the precision as float to avoid truncating from double.
This MR contains code-only fixes, and !13 (merged) is based on top of this MR.