SDL_shape.h: Non-namespaced enum

[circl-lastname]

The following is an enum in SDL_shape.h which does not respect the SDL_ prefix.

typedef enum {
    /** \brief The default mode, a binarized alpha cutoff of 1. */
    ShapeModeDefault,
    /** \brief A binarized alpha cutoff with a given integer value. */
    ShapeModeBinarizeAlpha,
    /** \brief A binarized alpha cutoff with a given integer value, but with the opposite comparison. */
    ShapeModeReverseBinarizeAlpha,
    /** \brief A color key is applied. */
    ShapeModeColorKey
} WindowShapeMode;

As long as the shape API is not removed, I believe this is a slight consistency issue.

This may be relevant to #6876 and #6569

[icculus]

I believe we're replacing this API, but I'm tossing this into the 3.2 milestone in case we don't.

[circl-lastname]

To close after #7316