[sdl2-image/mixer/net/ttf] Update to exported targets names.

[FtZPetruska]

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Description

Recently, a major CMake rework was done in the SDL2 subprojects, adding a consistent naming for the targets. Unfortunately, the naming chosen is inconsistent with the one used by vcpkg's ports.

The new naming consists in:

• The namespace SDL2_XXX:: where XXX is the name of the subproject
• A static target SDL2_XXX::SDL_XXX-static linked to SDL2::SDL2-static
• A dynamic target SDL2_XXX::SDL_XXX linked to SDL2::SDL2

Proposed solution

Adopt the naming chosen by the maintainers.

Alternatives

To avoid breaking existing projects/ports, the targets currently in use by vcpkg could be kept as an alias of the new one with a deprecation message, similar to what is currently done in SDL_ttf.

Additional context

Related PRs:

• Improve CMake build system libsdl-org/SDL_image#238
• Rewrite CMake build script libsdl-org/SDL_mixer#410
• Add CMake support libsdl-org/SDL_net#48
• Cmake work libsdl-org/SDL_ttf#213

[FtZPetruska]

Hello, I just wanted to update you about the upstream CMake changes.

The new target names have been shipped in the following stable releases:

• SDL2_image: version 2.6.x
• SDL2_mixer: version 2.6.x
• SDL2_ttf: version 2.20.x

In addition to the original issue, I noticed that the naming is also different between vcpkg's ports and the official releases when using find_package:

• vcpkg uses sdl2-XXX
• official releases use sdl2_XXX

[dg0yt]

Related: #25803