docs: code fixes for OpenGL ES training

code sample fixes reviewed separately,
see Change-Id: I1d760b75d1f2bfe1ec90c71471867577bd146241

fixing bugs:
b/10798358
b/10796990
b/10603728
b/7962328

Change-Id: I1e0f6668ec8d2b103b88c385f1f067d30ecc7178
10 files changed