Combine al_get_default_*_shader into al_get_default_shader_source.

The new name makes it clear that it returns the source rather than an
actual shader object, and would work for more more shader types if we
add any.

Also move the shader sources to a separate file.
Use ALLEGRO_SHADER_VAR macros in HLSL source.
ALLEGRO_SHADER_PLATFORM is an enum not a bitfield.
ex_shader: Simplify shader platform selection.