Try to clean up asset folders for sample apps. (#128)

* Try to clean up asset folders for sample apps.

This removes the build step where we copy a subset of assets, and makes
it so that FilamentApp hands out a "root path" for assets. For now this
is determined based on the location of the executable. This allows
developers to launch samples from any CWD.

Closes #11

* Restore asset copy to build.

samples/vk_texturedquad.cpp

@@ -81,14 +81,13 @@ int main() {
     auto setup = [&app](Engine* engine, View* view, Scene* scene) {
 
         // Load texture
-        Path path = Path::getCurrentExecutable().getParent() +
-                "textures/Moss_01/Moss_01_Color.png";
+        Path path = FilamentApp::getRootPath() + "third_party/textures/Moss_01/Moss_01_Color.png";
         if (!path.exists()) {
             std::cerr << "The texture " << path << " does not exist" << std::endl;
             exit(1);
         }
         int w, h, n;
-        unsigned char* data = stbi_load(path.getAbsolutePath().c_str(), &w, &h, &n, 4);
+        unsigned char* data = stbi_load(path.c_str(), &w, &h, &n, 4);
         if (data == nullptr) {
             std::cerr << "The texture " << path << " could not be loaded" << std::endl;
             exit(1);