gctx: disable scissor test before calling glClear()
gctx: disable scissor test before calling glClear()

The OpenGL documentation states:

> The pixel ownership test, the scissor test, dithering, and the buffer
writemasks affect the operation of glClear.

Without this patch, render targets can end up partially cleared if the
scissor test has been previously enabled.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl