compat/avisynth: update headers

As part of the update, it is now possible to test 32-bit GCC builds
of AviSynth+ with FFmpeg by using the AVS_WIN32_GCC32 define. Due to
different calling conventions between MSVC and GCC regarding 32-bit
Windows, this is unfortunately necessary.

compat/avisynth/avs/capi.h

@@ -39,17 +39,49 @@
 #  define EXTERN_C
 #endif
 
-#ifndef AVSC_USE_STDCALL
-#  define AVSC_CC __cdecl
-#else
-#  define AVSC_CC __stdcall
+#ifdef BUILDING_AVSCORE
+#  if defined(GCC) && defined(X86_32)
+#    define AVSC_CC
+#  else // MSVC builds and 64-bit GCC
+#    ifndef AVSC_USE_STDCALL
+#      define AVSC_CC __cdecl
+#    else
+#      define AVSC_CC __stdcall
+#    endif
+#  endif
+#else // needed for programs that talk to AviSynth+
+#  ifndef AVSC_WIN32_GCC32 // see comment below
+#    ifndef AVSC_USE_STDCALL
+#      define AVSC_CC __cdecl
+#    else
+#      define AVSC_CC __stdcall
+#    endif
+#  else
+#    define AVSC_CC
+#  endif
 #endif
 
+// On 64-bit Windows, there's only one calling convention,
+// so there is no difference between MSVC and GCC. On 32-bit,
+// this isn't true. The convention that GCC needs to use to
+// even build AviSynth+ as 32-bit makes anything that uses
+// it incompatible with 32-bit MSVC builds of AviSynth+.
+// The AVSC_WIN32_GCC32 define is meant to provide a user
+// switchable way to make builds of FFmpeg to test 32-bit
+// GCC builds of AviSynth+ without having to screw around
+// with alternate headers, while still default to the usual
+// situation of using 32-bit MSVC builds of AviSynth+.
+
+// Hopefully, this situation will eventually be resolved
+// and a broadly compatible solution will arise so the
+// same 32-bit FFmpeg build can handle either MSVC or GCC
+// builds of AviSynth+.
+
 #define AVSC_INLINE static __inline
 
 #ifdef BUILDING_AVSCORE
-#  define AVSC_EXPORT EXTERN_C
-#  define AVSC_API(ret, name) EXTERN_C __declspec(dllexport) ret AVSC_CC name
+#  define AVSC_EXPORT __declspec(dllexport)
+#  define AVSC_API(ret, name) EXTERN_C AVSC_EXPORT ret AVSC_CC name
 #else
 #  define AVSC_EXPORT EXTERN_C __declspec(dllexport)
 #  ifndef AVSC_NO_DECLSPEC

compat/avisynth/avs/config.h

@@ -42,7 +42,7 @@
 // alignment. They should always request the exact alignment value they need.
 // This is to make sure that plugins work over the widest range of AviSynth
 // builds possible.
-#define FRAME_ALIGN 32
+#define FRAME_ALIGN 64
 
 #if   defined(_M_AMD64) || defined(__x86_64)
 #   define X86_64
@@ -52,4 +52,19 @@
 #   error Unsupported CPU architecture.
 #endif
 
+#if   defined(_MSC_VER)
+#   define MSVC
+#elif defined(__GNUC__)
+#   define GCC
+#elif defined(__clang__)
+#   define CLANG
+#else
+#   error Unsupported compiler.
+#endif
+
+#if   defined(GCC)
+#   undef __forceinline
+#   define __forceinline inline
+#endif
+
 #endif //AVS_CONFIG_H

compat/avisynth/avs/types.h

@@ -35,6 +35,12 @@
 
 // Define all types necessary for interfacing with avisynth.dll
 
+#ifdef __cplusplus
+  #include <cstddef>
+#else
+  #include <stddef.h>
+#endif
+
 // Raster types used by VirtualDub & Avisynth
 typedef unsigned int    Pixel32;
 typedef unsigned char   BYTE;