Fix image scaling with masks (bug#38109)

Fix image scaling with masks (bug#38109)

* src/image.c (lookup_image): Move call to image_set_transform after
postprocess_image.
(image_create_x_image_and_pixmap_1): Use new function.
(image_set_transform): Apply the transform to the mask too.
(x_create_xrender_picture): New function.
(Create_Pixmap_From_Bitmap_Data):
(xpm_load): Use new function.
* src/xterm.c (x_composite_image): Use PictOpOver when there is a mask
so the transparency is honoured.
(x_draw_image_foreground_1): Use x_composite_image.