Use gen_struct_info for more things in library_sdl.js (#22626)

It looks like the hardcoded values here were actually wrong.

Author: sbc100

src/library_sdl.js

@@ -2668,19 +2668,19 @@ var LibrarySDL = {
 
 #if USE_SDL == 2
     if (rwops === undefined) {
-      var type = {{{ makeGetValue('rwopsID', 20 /*type*/, 'i32') }}};
+      var type = {{{ makeGetValue('rwopsID', C_STRUCTS.SDL_RWops.type, 'i32') }}};
 
       if (type === 2/*SDL_RWOPS_STDFILE*/) {
-        var fp = {{{ makeGetValue('rwopsID', 28 /*hidden.stdio.fp*/, 'i32') }}};
+        var fp = {{{ makeGetValue('rwopsID', C_STRUCTS.SDL_RWops.hidden.stdio.fp, 'i32') }}};
         var fd = _fileno(fp);
         var stream = FS.getStream(fd);
         if (stream) {
           rwops = { filename: stream.path };
         }
       }
       else if (type === 4/*SDL_RWOPS_MEMORY*/ || type === 5/*SDL_RWOPS_MEMORY_RO*/) {
-        var base = {{{ makeGetValue('rwopsID', 24 /*hidden.mem.base*/, 'i32') }}};
-        var stop = {{{ makeGetValue('rwopsID', 32 /*hidden.mem.stop*/, 'i32') }}};
+        var base = {{{ makeGetValue('rwopsID', C_STRUCTS.SDL_RWops.hidden.mem.base, 'i32') }}};
+        var stop = {{{ makeGetValue('rwopsID', C_STRUCTS.SDL_RWops.hidden.mem.stop, 'i32') }}};
 
         rwops = { bytes: base, count: stop - base };
       }

src/struct_info.json

@@ -705,6 +705,29 @@
             ]
         }
     },
+    {
+        "file": "SDL/SDL_rwops.h",
+        "structs": {
+            "SDL_RWops": [
+                "type",
+                {
+                  "hidden": [
+                    {
+                      "mem": [
+                         "base",
+                         "stop"
+                      ]
+                    },
+                    {
+                      "stdio": [
+                         "fp"
+                      ]
+                    }
+                  ]
+                }
+            ]
+        }
+    },
     {
         "file": "uuid/uuid.h",
         "defines": [

src/struct_info_generated.json

@@ -744,6 +744,22 @@
             "format": 0,
             "palette": 4
         },
+        "SDL_RWops": {
+            "__size__": 32,
+            "hidden": {
+                "__size__": 12,
+                "mem": {
+                    "__size__": 12,
+                    "base": 20,
+                    "stop": 28
+                },
+                "stdio": {
+                    "__size__": 8,
+                    "fp": 24
+                }
+            },
+            "type": 16
+        },
         "SDL_Rect": {
             "__size__": 16,
             "h": 12,

src/struct_info_generated_wasm64.json

@@ -744,6 +744,22 @@
             "format": 0,
             "palette": 8
         },
+        "SDL_RWops": {
+            "__size__": 64,
+            "hidden": {
+                "__size__": 24,
+                "mem": {
+                    "__size__": 24,
+                    "base": 40,
+                    "stop": 56
+                },
+                "stdio": {
+                    "__size__": 16,
+                    "fp": 48
+                }
+            },
+            "type": 32
+        },
         "SDL_Rect": {
             "__size__": 16,
             "h": 12,