Fix clang++ compilation errors

clang was complaining about the type conversions. For example...
```
node_modules/raylib-src/src/core.c:1888:15: error: cannot initialize a variable of type 'char *' with an
      rvalue of type 'void *'
        char *cmd = calloc(strlen(url) + 10, sizeof(char));
              ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

src/core.c

@@ -276,7 +276,7 @@ static bool windowMinimized = false;            // Check if window has been mini
 static const char *windowTitle = NULL;          // Window text title...
 
 static unsigned int displayWidth, displayHeight;// Display width and height (monitor, device-screen, LCD, ...)
-static int screenWidth, screenHeight;           // Screen width and height (used render area)
+//static int screenWidth, screenHeight;           // Screen width and height (used render area)
 static int renderWidth, renderHeight;           // Framebuffer width and height (render area, including black bars if required)
 static int renderOffsetX = 0;                   // Offset X from render area (must be divided by 2)
 static int renderOffsetY = 0;                   // Offset Y from render area (must be divided by 2)
@@ -1642,7 +1642,7 @@ const char *GetFileNameWithoutExt(const char *filePath)
 
     // Try to allocate new string, same size as original
     // NOTE: By default strlen() does not count the '\0' character
-    if ((result = malloc(strlen(filePath) + 1)) == NULL) return NULL;
+    if ((result = (char *)malloc(strlen(filePath) + 1)) == NULL) return NULL;
 
     strcpy(result, filePath);   // Make a copy of the string
 
@@ -1885,7 +1885,7 @@ void OpenURL(const char *url)
     }
     else
     {
-        char *cmd = calloc(strlen(url) + 10, sizeof(char));
+        char *cmd = (char *)calloc(strlen(url) + 10, sizeof(char));
 
 #if defined(_WIN32)
         sprintf(cmd, "explorer '%s'", url);