Don't use the hardcoded indices 0 and 1 but the ones returned from
QOpenGLShaderProgram::attributeLocation(). Also change them from GLuint
to GLint.
Fixes: QTBUG-43566
Change-Id: I582bcf899f20d90f4ec1cd61326a177aabb69992
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Laszlo Agocs <laszlo.agocs@qt.io>