GLProducer: Reference count images rather than buffers.

In most cases, EGLImages can be created one-to-one with graphic
buffers in slots, but that was difficult due to some special
cases:
- ReleaseTexImage binds a custom 'unslotted' debug image.
- When all slots are freed, we still need to hang on to one.

These cases were handled by keeping an additional reference to
the 'current' buffer (mCurrentTextureBuf), but we would create
new images since we can't reference count them in the same way.
This patch uses the same semantics, except that it reference
counts the image (an EglImage wrapper class) rather than just
buffer. The wrapper class also detects the cases when we need
a new EGLImage, and only creates them in those rare cases.

Change-Id: I2915761dbe49d2a9bda1f59e60f857543634636b