Embed the SDL2 libraries into vendor:sdl2

vendor/sdl2/image/LICENSE.tiff.txt

@@ -0,0 +1,25 @@
+The source code to this library used with SDL_image can be found here:
+https://hg.libsdl.org/SDL_image/file/default/external
+---
+
+Copyright (c) 1988-1997 Sam Leffler
+Copyright (c) 1991-1997 Silicon Graphics, Inc.
+
+Permission to use, copy, modify, distribute, and sell this software and 
+its documentation for any purpose is hereby granted without fee, provided
+that (i) the above copyright notices and this permission notice appear in
+all copies of the software and related documentation, and (ii) the names of
+Sam Leffler and Silicon Graphics may not be used in any advertising or
+publicity relating to the software without the specific, prior written
+permission of Sam Leffler and Silicon Graphics.
+
+THE SOFTWARE IS PROVIDED "AS-IS" AND WITHOUT WARRANTY OF ANY KIND, 
+EXPRESS, IMPLIED OR OTHERWISE, INCLUDING WITHOUT LIMITATION, ANY 
+WARRANTY OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.  
+
+IN NO EVENT SHALL SAM LEFFLER OR SILICON GRAPHICS BE LIABLE FOR
+ANY SPECIAL, INCIDENTAL, INDIRECT OR CONSEQUENTIAL DAMAGES OF ANY KIND,
+OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
+WHETHER OR NOT ADVISED OF THE POSSIBILITY OF DAMAGE, AND ON ANY THEORY OF 
+LIABILITY, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE 
+OF THIS SOFTWARE.