Refactor SkGLContext to be actually extendable

Refactor SkGLContext to be actually extendable. Before, non-trivial subclass
would need to destroy the GL connection upon running the destructor. However,
the base class would run GL commands in its own destructor (with destroyed GL
connection)

Refactor so that SkGLContext subclass object creation is completely done by
the factory function. If the factory function returns a non-NULL ptr, it means the context
is usable.

The destruction is done with the destructor instead of virtual function called
upon destruction. Make the destructors not to call virtual functions, for
clarity.

Remove custom 1x1 FBO setup code from the base class. It appears not to be used
anymore.

BUG=skia:2992

Review URL: https://codereview.chromium.org/640283004