QOpenGLTextureHelper: fix the DSA emulation

The DSA emulator functions query which texture is currently bound
to a given target, then bind the new one, perform an operation,
and bind the old one back.

The problem is that in order to query what's currently bound to the
GL_TEXTURE_<X> target one needs to call glGetIntegerv passing
GL_BINDING_TEXTURE_<X>. Since both GL_TEXTURE_X and GL_BINDING_TEXTURE_X
values are completely arbitrary (not contiguous nor related in any way)
we need to pass *both* them to the functions.

The right GL_BINDING_TEXTURE_X was getting already chosen (and stored)
at texture creation time by QOpenGLTexture, so it's just a matter
of passing it around.

For the "real" DSA functions, the binding target is ignored.

Change-Id: Ida823abbfb142d4a22bf9f9a762b160b7e281c6d
Reviewed-by: James Turner <james.turner@kdab.com>
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>
QOpenGLTexture: remove typos in the docs

Change-Id: Ia842c7f91eadefc6b5328d7d951c4e265c2c7432
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>