Added some missing functions and flip-Y unpack extension.

Listed all missing functions and implemented drawingBufferWidth(), drawingBufferHeight(), getActiveAttrib(), getActiveUniform() functions.
Removed default Y-flip and implemented support for UNPACK_FLIP_Y_WEBGL, this breaks functionality but now behavior is consistent with the spec.

Change-Id: Ie6ff879f5b7acea713ab4729eccc6c8948ad394f
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>