Return proper viewport rect for render target.

When drawing the content of a RenderTargetPanel to a texture, the panel's
position in the GUI should not affect the position of the viewport.
Remove obsolete comment.

Resources may not be pruned at random if they're still in use.
Use gcc 4.8 when building for Android.

Android NDK version r9 threw out gcc 4.7.
Update README.
Use enums for identifying uniforms and attributes.

Avoids multiple string comparisons everytime a material or model is prepared for
rendering.
Make sure destructors get called.

Adds missed virtual destructors and forgotten class final keywords.
Add Audio module.

The Audio module is the backend performing audio playback, currently using
OpenAL or OpenSLES depending on platform.
Add libogg and tremor external libraries.

For Ogg Vorbis decoding.
Add Sound module.

Lets game code play audio files. Currently only Ogg Vorbis is supported.
Update README.