sdl2: security bump to version 2.0.7

Fixes CVE-2017-2888 - An exploitable integer overflow vulnerability exists
when creating a new RGB Surface in SDL 2.0.5.  A specially crafted file can
cause an integer overflow resulting in too little memory being allocated
which can lead to a buffer overflow and potential code execution.  An
attacker can provide a specially crafted image file to trigger this
vulnerability.

Also add a hash for the license file while we're at it.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>