Move generated directory to build directory

StepMania generates some files during build but it's a bit unusual to generate
them in source directory instead of build directory. Move them to build
directory and remove src/generated from .gitignore.

src/CMakeProject-texture.cmake

@@ -44,7 +44,7 @@ target_link_libraries("TextureFontGenerator" ${TEXTURE_LINK_LIB})
 list(APPEND TEXTURE_INCLUDE_DIRS
             "${TEXTURE_DIR}"
             "${TEXTURE_DIR}/res"
-            "${SM_SRC_DIR}/generated")
+            "${SM_GENERATED_SRC_DIR}")
 
 target_include_directories("TextureFontGenerator"
                            PUBLIC ${TEXTURE_INCLUDE_DIRS})