Modify the definition of SDImageFormat to extern to avoid creating multiple global variables #3699
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I found that several formats about SDImageFormat are static variables defined in a header file. When the program is compiled and linked, any .m file that imports or indirectly imports this header will create a static variable inside the file. This results in an increase in useless binary size. In fact, only need to declare these variables as extern and assign values inside a certain .m file, so that there will only be one copy of these variables in the entire binary.