Transparent background in exported .png images about fluidx3d

[ProjectPhysX]

As-is, .png export only supports the 3-channel .png format in favor of smaller file size. But you can easily change that:

1. in src/defines.hpp, set GRAPHICS_BACKGROUND_COLOR to 0xFF000000
2. in src/utilities.hpp starting at line 4133, replace the entire write_png(...) function with this code:

inline void write_png(const string& filename, const Image* image) {
	create_folder(filename);
	uchar* data = new uchar[4u*image->length()];
	for(uint i=0u; i<image->length(); i++) {
		const int color = image->color(i);
		data[4u*i   ] = (color>>16)&255;
		data[4u*i+1u] = (color>> 8)&255;
		data[4u*i+2u] =  color     &255;
		data[4u*i+3u] = 255-((color>>24)&255);
	}
	lodepng::encode(create_file_extension(filename, ".png"), data, image->width(), image->height(), LCT_RGBA);
	delete[] data;
}