shader: Drop Cg backend.

It will give us one less backend to maintain for now.
It can be brought back later if someone wants.
Move shader sources to core library.

The headers still need some work.
Finish migrating shader headers to the core.

Since there is no more shader addon, allegro_shader.h no longer exists
for user programs to include.

Rename allegro_shader_{glsl,hlsl}.h to allegro_{glsl,hlsl}.h.
Move shader variable name macros to shader.h.
docs: Move shader API to main section.
ex_palette: Use al_set_shader instead of al_set_opengl_program_object.
Remove al_get_opengl_program_object, al_set_opengl_program_object.

With the shader API in the core library they are no longer necessary.
According to Trent, its original purpose was so you could use a complete
programmable pipeline with Allegro without having to link the shader
addon.
Remove al_get_direct3d_effect, al_set_direct3d_effect.

As with the GL backend, these are no longer necessary with the shader
API in the core.
al_create_shader(ALLEGRO_SHADER_AUTO) should not try GLSL backend for
non-OpenGL displays if HLSL backend is disabled.
Log error if D3DXCreateEffect is unavailable.
primitives: Fix use of uninitialised variable.