Support installing demos, support out-of-tree builds

This is especially useful for cross-compile situation where testing
happens on target.

-DRESOURCE_INSTALL_DIR=<path> decides where data is installed (and
where the binaries will load the data from): if it's left empty,
then nothing will be installed and binaries will load the data from
CMAKE_SOURCE_DIR.

Binaries are now correctly built in CMAKE_BINARY_DIR.

base/vulkanexamplebase.cpp

@@ -83,6 +83,8 @@ const std::string VulkanExampleBase::getAssetPath()
 {
 #if defined(__ANDROID__)
 	return "";
+#elif defined(VK_EXAMPLE_DATA_DIR)
+	return VK_EXAMPLE_DATA_DIR;
 #else
 	return "./../data/";
 #endif