Updated to version 3.1.5 for development

We should bump the version immediately before and after release so the release version corresponds to a fixed snapshot of code.

CMakeLists.txt

@@ -5,7 +5,7 @@ if(CMAKE_CURRENT_SOURCE_DIR STREQUAL CMAKE_CURRENT_BINARY_DIR)
 endif()
 
 # See docs/release_checklist.md
-project(SDL3 LANGUAGES C VERSION "3.1.3")
+project(SDL3 LANGUAGES C VERSION "3.1.5")
 
 if(CMAKE_SOURCE_DIR STREQUAL PROJECT_SOURCE_DIR)
   set(SDL3_SUBPROJECT OFF)