* Get C++ compilers working

* Fix Formatting

- make sure that src/external/rl_gputex.h uses it's own macros
    for printing warnings and allocating memory
- add few additional macros in order to decouple rl_gputex from headers
    that are being included by Raylib (e.g. *_MEMSET, *_NULL and so on)
- make sure that rl_gputex uses RL_*ALLOC, TRACELOG and RLAPI macros
    when (and only when) being included by src/rtextures.c
- replace LOG() and RL_GPUTEX_SHOW_LOG_INFO macros with: RL_GPUTEX_WARN
    and RL_GPUTEX_SHOW_WARN_INFO (this is a breaking change, but it was
    broken and unusable anyway, see:
    https://github.com/raysan5/raylib/issues/5039#issuecomment-3065732596 )
- fixes issue all issues mentioned in:
    https://github.com/raysan5/raylib/issues/5039#issuecomment-3065732596
- remove logging upon successfully saving a file; Raylib does this
    anyway and we want rl_gputex to only print WARNings upon failures;
    see: https://discord.com/channels/426912293134270465/541710686468702250/1394406734306480352
- add additional condition when saving a file; now checks for both
    fwrite() and fclose() failures; before it would warn about both,
    but only return result based on fclose()
- add some notes about the current state of compiling rl_gputex
    without depending on Raylib (spoiler: it's still broken...)
- bump rl_gputex version to 1.1 since this is a potential breaking
    change (only for people attempting to use it without Raylib)

Fixes: https://github.com/raysan5/raylib/issues/5039
Reference: https://discord.com/channels/426912293134270465/541710686468702250/1394403611852931255

* Fix HalfToFloat() and FloatToHalf() dereferencing issues with an union

* Remove unnecessary initialization

* Moved the union to inside the functions

* [rtextures] Only build cubemap mipmaps when necessary

* [rtextures] Assign correct mipmap count to cubemaps

define STBIR_NO_SIMD when __TINYC__ is defined so stb_image_resize2 will
not include *mmintrin which are not supported by all compilers.

There are similar checks for __TINYC__ already elswere in raylib
and they are also mostly there to disable SIMD headers.

Additionally, move similar check for stb_image, to be a little bit
deeper. Before it was defining STBI_NO_SIMD without including stb_image
It was also clashing with note, causing said note to make no sense.

Fixes: https://github.com/raysan5/raylib/discussions/2994
Reference: https://github.com/nothings/stb/issues/1738

* [rlgl] Load cubemap mipmaps

* [rtextures] Only generate mipmaps that don't already exist

* [rtextures] ImageDraw(): Implement drawing to mipmaps

* [rtextures] Load cubemap mipmaps

* Remove the panorama cubemap layout, it was not implemented.
Left a todo in the code for some aspiring developer to finish.

* Update raylib_api.* by CI

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