Reimplement clipboard text in terms of clipboard data

This will simplify the X11 and Wayland implementations, which were doing that under the hood, and makes application interaction between the two APIs consistent.

include/SDL3/SDL_clipboard.h

@@ -171,6 +171,8 @@ typedef void (SDLCALL *SDL_ClipboardCleanupCallback)(void *userdata);
  * data the callback function will be called allowing it to generate and
  * respond with the data for the requested mime-type.
  *
+ * The size of text data does not include any terminator, and the text does not need to be null terminated (e.g. you can directly copy a portion of a document)
+ *
  * \param callback A function pointer to the function that provides the
  *                 clipboard data
  * \param cleanup A function pointer to the function that cleans up the
@@ -202,6 +204,8 @@ extern DECLSPEC int SDLCALL SDL_ClearClipboardData();
 /**
  * Get the data from clipboard for a given mime type
  *
+ * The size of text data does not include the terminator, but the text is guaranteed to be null terminated.
+ *
  * \param mime_type The mime type to read from the clipboard
  * \param size      A pointer filled in with the length of the returned data
  * \returns the retrieved data buffer or NULL on failure; call SDL_GetError()