Add al_get_shader_platform.

This makes it a little simpler/direct to specify what type of source file you will be passing to al_attach_shader_source. Previously you had to examine the current display flags which was roundabout at best.
Make ex_prim_shader use al_get_shader_platform
Check that the shader creation succeeded in ex_prim_shader
Move the shader in ex_shader_multitex into a separate file
Remove the last vestiges of the Cg shader backend.
Add default HLSL shaders
Modify ex_prim to use the default shaders (to test them) when --shader switch is used.
Implement the HLSL shader for ex_shader_multitex
ex_shader was using the wrong macros to detect Direct3D