[AD] shader addon questions

[Elias Pschernig <elias.pschernig@xxxxxxxxxx>]

I tried documenting the shader addon a bit but there's a few things
I've been wondering about.

1. What does al_set_shader do? Shouldn't al_use_shader be enough?

2. what is the difference between these three functions?

al_set_shader_vertex_array
al_set_shader_color_array
al_set_shader_texcoord_array

Looking at the OpenGL source code they all just seem to call
glVertexAttrib so shouldn't we just have an al_set_vertex_attribute
function instead?

3. al_set_shader_float_vector

If I understood correctly, this either sets an *array of floats* if
elem_size is 1 or alternatively an *array of float vectors* if
elem_size is 2, 3 or 4. So the name is misleading. My suggestion would
be to rename it to al_set_shader_float_array (since
al_set_shader_array_of_float_vectors would be a bit long).