[rl_gputex] fix: decouple logging and memory allocation from Raylib

- 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

src/rtextures.c

@@ -168,6 +168,18 @@
         #pragma GCC diagnostic push
         #pragma GCC diagnostic ignored "-Wunused-function"
     #endif
+ 
+
+    #define RL_GPUTEX_MALLOC     RL_MALLOC
+    #define RL_GPUTEX_CALLOC     RL_CALLOC
+    #define RL_GPUTEX_REALLOC    RL_REALLOC
+    #define RL_GPUTEX_FREE       RL_FREE
+    #define RL_GPUTEX_WARN(...)  TRACELOG(LOG_WARNING, "IMAGE: " __VA_ARGS__)
+    #define RL_GPUTEX_SHOW_WARN_INFO
+
+    // FIXME: probably, we should NOT export public functions from rl_gputex
+    //        but this is how it always worked... so let's keep it this way
+    #define RLGPUTEXAPI RLAPI
 
     #define RL_GPUTEX_IMPLEMENTATION
     #include "external/rl_gputex.h"         // Required for: rl_load_xxx_from_memory()