change window shape with SDL_SetWindowShape

[lpalozzi]

Hi,

Trying to change the window shape after previously being set with SDL_SetWindowShape, but it seems to do a union with previous shapes.

Following X11_SetWindowShape and SDL_CalculateShapeBitmap, the bitmap shaper->driverdata is not zeroed, and previous bitmap is OR'd with the new shape. X11_XShapeCombineMask is then called with ShapeSet operation.

I guess the Shape Extension is not fully supported?

Thanks.

[lpalozzi]

In SDL_CalculateShapeBitmap I zero the bitmap to fix it in my case. I'm not sure if this is a general solution for all use cases.

  if(SDL_MUSTLOCK(shape))
       SDL_LockSurface(shape);

   SDL_memset(bitmap, 0, shape->h * bytes_per_scanline);

   for(y = 0;y<shape->h;y++) {

[slouken]

This change looks good, thanks!