Make allegro_glsl.h not imply allegro_opengl.h.

This introduced compilation errors on ex_prim.c once it included
allegro_glsl.h which includes allegro_opengl.h which, ..., which
includes Xlib.h which defines Screen and Font, which conflict with
ex_prim.c.
ex_prim: Conditionally call GLSL/HLSL-specific functions.
ex_shader_multitex: Conditionally call GLSL/HLSL-specific functions.
test_driver: Support HLSL shader.