Address type conversion/truncation warnings in LoadPcxPixelsAndPalette

SDL_RWsize is documented as returning any negative value on error (even though in practice errors are always indicated by a value of -1) so allow for that instead of only checking for unknown size.

There were also a few unnecessary arithmetic operations being done to calculate the size to read, straightforward to simplify.

SDL_CreateRGBSurfaceWithFormat expects width and height to be positive values < 2^16. Could get away with using unsigned if the SDL API did but it doesn't so stick with signed types even when it doesn't really fit the use.

bufferPitch is an oddity, ultimately it's only used to determine a pointer offset so making the param ptrdiff_t as it's more appropriate than size_t.

Source/DiabloUI/art.cpp

@@ -15,7 +15,6 @@ namespace devilution {
 namespace {
 constexpr size_t PcxHeaderSize = 128;
 constexpr unsigned NumPaletteColors = 256;
-constexpr unsigned PcxPaletteSize = 1 + NumPaletteColors * 3;
 
 bool LoadPcxMeta(SDL_RWops *handle, int &width, int &height, std::uint8_t &bpp)
 {
@@ -30,23 +29,23 @@ bool LoadPcxMeta(SDL_RWops *handle, int &width, int &height, std::uint8_t &bpp)
 }
 
 bool LoadPcxPixelsAndPalette(SDL_RWops *handle, int width, int height, std::uint8_t bpp,
-    uint8_t *buffer, std::size_t bufferPitch, SDL_Color *palette)
+    uint8_t *buffer, std::ptrdiff_t bufferPitch, SDL_Color *palette)
 {
-	const bool has256ColorPalette = palette != nullptr && bpp == 8;
-	std::uint32_t pixelDataSize = SDL_RWsize(handle);
-	if (pixelDataSize == static_cast<std::uint32_t>(-1)) {
+	std::ptrdiff_t pixelDataSize = SDL_RWsize(handle);
+	if (pixelDataSize < 0) {
+		// Unable to determine size, or an error occurred.
 		return false;
 	}
-	pixelDataSize -= PcxHeaderSize + (has256ColorPalette ? PcxPaletteSize : 0);
 
-	// We read 1 extra byte because it delimits the palette.
-	const size_t readSize = pixelDataSize + (has256ColorPalette ? PcxPaletteSize : 0);
+	// SDL_RWsize gives the total size of the file however we've already read the header from an earlier call to
+	//  LoadPcxMeta, so we only need to read the remainder of the file.
+	const std::size_t readSize = pixelDataSize - PcxHeaderSize;
 	std::unique_ptr<uint8_t[]> fileBuffer { new uint8_t[readSize] };
 	if (SDL_RWread(handle, fileBuffer.get(), readSize, 1) == 0) {
 		return false;
 	}
-	const unsigned xSkip = bufferPitch - width;
-	const unsigned srcSkip = width % 2;
+	const std::ptrdiff_t xSkip = bufferPitch - width;
+	const std::ptrdiff_t srcSkip = width % 2;
 	uint8_t *dataPtr = fileBuffer.get();
 	for (int j = 0; j < height; j++) {
 		for (int x = 0; x < width;) {
@@ -67,9 +66,10 @@ bool LoadPcxPixelsAndPalette(SDL_RWops *handle, int width, int height, std::uint
 		buffer += xSkip;
 	}
 
-	if (has256ColorPalette) {
+	if (palette != nullptr && bpp == 8) {
+		// The file has a 256 color palette that needs to be loaded.
 		[[maybe_unused]] constexpr unsigned PcxPaletteSeparator = 0x0C;
-		assert(*dataPtr == PcxPaletteSeparator);
+		assert(*dataPtr == PcxPaletteSeparator); // sanity check the delimiter
 		++dataPtr;
 
 		auto *out = palette;