Added QTCANVAS3D_gl_state_dump extension for dumping the current GL state for debugging purposes and added example use to jsonmodels example.
Fixed documentation issues and removed invalid comments related to render thread.

Change-Id: Ib4a81370bf2673a5bfb412038148646638f723a9
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>

QQmlPropertyMap as baseclass doesn't work without additional work, it has been now removed as baseclass from QtCanvas3D for now.
TextureLoader has been removed in favor of TextureImageFactory as that allows easier porting of WebGL based JavaScript code to QtCanvas3D.

Change-Id: I6fb374fd012d7569b9f173b08f4a6f21dcd83abe
Reviewed-by: Tomi Korpipää <tomi.korpipaa@digia.com>
Reviewed-by: Pasi Keränen <pasi.keranen@digia.com>

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>