SurfaceTexture: make tests use a large framebuffer.

This change makes the SurfaceTexture GL tests use a large (512x512)
framebuffer.  The tests now use glViewport to render to a region of the
framebuffer with a size matching that of the texture being drawn.  The
Npot test pixel checks must be changed because now the rendered region
is the same size as the texture.

Change-Id: Icc41803189c25e33ba1da7d2470720f0a11f3e4b