Fix `SBmpLoadImage` size allocation

SBmpLoadImage tried to get the size of the rest of the file incorrectly
by subtracting the current file position from the total size.

This was incorrect because file positions do not always begin at 0.

The only semi-portable way to get a size from file positions is to
subtract two file positions.

3rdParty/Storm/Source/storm.h

@@ -346,7 +346,7 @@ bool SNetRegisterEventHandler(event_type, SEVTHANDLER);
 BOOLEAN SNetSetBasePlayer(int);
 int SNetInitializeProvider(unsigned long, struct _SNETPROGRAMDATA *, struct _SNETPLAYERDATA *, struct _SNETUIDATA *, struct _SNETVERSIONDATA *);
 int SNetGetProviderCaps(struct _SNETCAPS *);
-int SFileSetFilePointer(HANDLE, int, HANDLE, int);
+int SFileSetFilePointer(HANDLE, int, int*, int);
 BOOL SFileEnableDirectAccess(BOOL enable);
 
 #if defined(__GNUC__) || defined(__cplusplus)

SourceX/storm/storm.cpp

@@ -1,4 +1,5 @@
 #include <cstddef>
+#include <cstdint>
 #include <string>
 
 #include "all.h"
@@ -126,6 +127,23 @@ BOOL SFileOpenFile(const char *filename, HANDLE *phFile)
 	return result;
 }
 
+static std::uint64_t GetFilePointer(HANDLE hFile)
+{
+    // We use `SFileSetFilePointer` with offset 0 to get the current position
+    // because there is no `SFileGetFilePointer`.
+    int high = 0;
+    const std::uint32_t low = SFileSetFilePointer(hFile, 0, &high, DVL_FILE_CURRENT);
+    return (static_cast<std::uint64_t>(high) << 32) | low;
+}
+
+static std::uint64_t SetFilePointer(HANDLE hFile, std::int64_t offset, int whence)
+{
+    int high = static_cast<std::uint64_t>(offset) >> 32;
+    int low = static_cast<int>(offset);
+    low = SFileSetFilePointer(hFile, low, &high, whence);
+    return (static_cast<std::uint64_t>(high) << 32) | low;
+}
+
 BOOL SBmpLoadImage(const char *pszFileName, SDL_Color *pPalette, BYTE *pBuffer, DWORD dwBuffersize, DWORD *pdwWidth, DWORD *dwHeight, DWORD *pdwBpp)
 {
 	HANDLE hFile;
@@ -192,10 +210,11 @@ BOOL SBmpLoadImage(const char *pszFileName, SDL_Color *pPalette, BYTE *pBuffer,
 		*pdwBpp = pcxhdr.BitsPerPixel;
 
 	if (!pBuffer) {
-		SFileSetFilePointer(hFile, 0, 0, DVL_FILE_END);
+		SFileSetFilePointer(hFile, 0, NULL, DVL_FILE_END);
 		fileBuffer = NULL;
 	} else {
-		size = SFileGetFileSize(hFile, 0) - SFileSetFilePointer(hFile, 0, 0, DVL_FILE_CURRENT);
+		const auto pos = GetFilePointer(hFile);
+		size = SetFilePointer(hFile, 0, DVL_FILE_END) - SetFilePointer(hFile, pos, DVL_FILE_BEGIN);
 		fileBuffer = (BYTE *)malloc(size);
 	}
 
@@ -228,7 +247,7 @@ BOOL SBmpLoadImage(const char *pszFileName, SDL_Color *pPalette, BYTE *pBuffer,
 	}
 
 	if (pPalette && pcxhdr.BitsPerPixel == 8) {
-		SFileSetFilePointer(hFile, -768, 0, 1);
+		SFileSetFilePointer(hFile, -768, NULL, DVL_FILE_CURRENT);
 		SFileReadFile(hFile, paldata, 768, 0, 0);
 		for (int i = 0; i < 256; i++) {
 			pPalette[i].r = paldata[i][0];