glActiveTexture should precede glBindTexture
Don't clobber the 0'th texture unit in OpenGL

Previously, when we were using shaders and when texture was NULL, the bitmap
would be set to 0 always, which broke shader setups which tried to use the 0'th
texture unit.
Don't clobber the 0'th texture unit in D3D

See previous commit.
Document how al_set_shader_sampler interacts with the `al_tex`