Custom implementation that returns -1 if it fails (no negative values supported)

...while using a RenderTexture combined with 3d mode

REDESIGNED: rlLoadRenderTexture()
ADDED: rlRenderTextureAttach()
ADDED: rlRenderTextureComplete()

Probably uncomplete, not tested yet...

Getting the following strict error....

```
src/text.c:117:105: error: constant expression evaluates to 2398692640 which
      cannot be narrowed to type 'int' [-Wc++11-narrowing]
...0x00000000, 0x00000000, 0x00200020, 0x0001b000, 0x00000000, 0x00000000, 0x8ef92520, 0x00020a00...
                                                                             ^~~~~~~~~~
```

Switching it to an unsigned int fixes it.

clang was complaining about the type conversions. For example...
```
node_modules/raylib-src/src/core.c:1888:15: error: cannot initialize a variable of type 'char *' with an
      rvalue of type 'void *'
        char *cmd = calloc(strlen(url) + 10, sizeof(char));
              ^     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Function provisional renaming... that's not the end of this issue...

When capturing framebuffer, if some element with transparency has been drawn, it retrieves that data... it could be a bit annoying in some situations because we could expect color compositing with background color... It depends on the situation and our needs... but most of the time we don't want that transparency on screenshots.

Following raylib notation rules

This specific change could be very annoying for people already using FLAG_MSAA_4X_HINT, their programs will compile correctly but window won't be shown...