Wayland: Fix partial writes of clipboard string

The string pointer used to write the contents of our clipboard data
offer was never updated, causing it to repeat parts of the beginning of
the string until the correct number of bytes had been written.

README.md

@@ -313,6 +313,7 @@ information on what to include when reporting a bug.
  - [Wayland] Bugfix: `glfwSetClipboardString` would fail if set to result of
    `glfwGetClipboardString`
  - [Wayland] Bugfix: Data source creation error would cause double free at termination
+ - [Wayland] Bugfix: Partial writes of clipboard string would cause beginning to repeat
  - [POSIX] Removed use of deprecated function `gettimeofday`
  - [POSIX] Bugfix: `CLOCK_MONOTONIC` was not correctly tested for or enabled
  - [WGL] Disabled the DWM swap interval hack for Windows 8 and later (#1072)

src/wl_window.c

@@ -1591,7 +1591,7 @@ static void dataSourceHandleSend(void* data,
                                  const char* mimeType,
                                  int fd)
 {
-    const char* string = _glfw.wl.clipboardSendString;
+    char* string = _glfw.wl.clipboardSendString;
     size_t len = strlen(string);
     int ret;
 
@@ -1631,6 +1631,7 @@ static void dataSourceHandleSend(void* data,
             return;
         }
         len -= ret;
+        string += ret;
     }
     close(fd);
 }