Update 'SDL2' package to v2.0.18 (#502)

Author: drodin

cmake/configs/default.cmake

@@ -143,7 +143,7 @@ hunter_default_version(Qwt VERSION 6.1-p3)
 hunter_default_version(RapidJSON VERSION 1.1.0-b557259-p0)
 hunter_default_version(RapidXML VERSION 1.13)
 hunter_default_version(RedisClient VERSION 0.6.1-p1)
-hunter_default_version(SDL2 VERSION 2.0.16-p0)
+hunter_default_version(SDL2 VERSION 2.0.18)
 hunter_default_version(SDL_image VERSION 2.0.5-p0)
 hunter_default_version(SDL_mixer VERSION 2.0.4-p0)
 hunter_default_version(SDL_net VERSION 2.0.1-p0)

cmake/projects/SDL2/hunter.cmake

@@ -149,6 +149,17 @@ hunter_add_version(
         3b8d0dfdbd1b062035a9432bff75a22ab439bcd1
 )
 
+hunter_add_version(
+        PACKAGE_NAME
+        SDL2
+        VERSION
+        "2.0.18"
+        URL
+        "https://github.com/libsdl-org/SDL/archive/release-2.0.18.tar.gz"
+        SHA1
+        b765ac6a7bfc617ce51ff2571b1ae056357413f2
+)
+
 hunter_add_version(
         PACKAGE_NAME
         SDL2