Functions which return function pointers now return SDL_FunctionPointer instead of void*

This fixes the clang warning "Cast between pointer-to-function and pointer-to-object is an extension"

You can define SDL_FUNCTION_POINTER_IS_VOID_POINTER in your project to restore the previous behavior.

Fixes https://github.com/libsdl-org/SDL/issues/2866

src/loadso/dlopen/SDL_sysloadso.c

@@ -32,8 +32,7 @@
 #include "../../video/uikit/SDL_uikitvideo.h"
 #endif
 
-void *
-SDL_LoadObject(const char *sofile)
+void *SDL_LoadObject(const char *sofile)
 {
     void *handle;
     const char *loaderror;
@@ -53,8 +52,7 @@ SDL_LoadObject(const char *sofile)
     return handle;
 }
 
-void *
-SDL_LoadFunction(void *handle, const char *name)
+SDL_FunctionPointer SDL_LoadFunction(void *handle, const char *name)
 {
     void *symbol = dlsym(handle, name);
     if (symbol == NULL) {

src/loadso/dummy/SDL_sysloadso.c

@@ -25,16 +25,14 @@
 /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
 /* System dependent library loading routines                           */
 
-void *
-SDL_LoadObject(const char *sofile)
+void *SDL_LoadObject(const char *sofile)
 {
     const char *loaderror = "SDL_LoadObject() not implemented";
     SDL_SetError("Failed loading %s: %s", sofile, loaderror);
     return NULL;
 }
 
-void *
-SDL_LoadFunction(void *handle, const char *name)
+SDL_FunctionPointer SDL_LoadFunction(void *handle, const char *name)
 {
     const char *loaderror = "SDL_LoadFunction() not implemented";
     SDL_SetError("Failed loading %s: %s", name, loaderror);

src/loadso/windows/SDL_sysloadso.c

@@ -27,8 +27,7 @@
 
 #include "../../core/windows/SDL_windows.h"
 
-void *
-SDL_LoadObject(const char *sofile)
+void *SDL_LoadObject(const char *sofile)
 {
     void *handle;
     LPTSTR tstr;
@@ -59,8 +58,7 @@ SDL_LoadObject(const char *sofile)
     return handle;
 }
 
-void *
-SDL_LoadFunction(void *handle, const char *name)
+SDL_FunctionPointer SDL_LoadFunction(void *handle, const char *name)
 {
     void *symbol = (void *)GetProcAddress((HMODULE)handle, name);
     if (symbol == NULL) {