[AD] Some more thoughts on the shader API

[SiegeLord <slabode@xxxxxxxxxx>]

So after trying to use the current API some more while working on the 
examples I had some other ideas about simplifying it. One thing I 
noticed was that the default shaders were just very verbose to use. 
Firstly, you had to include both allegro_hlsl.h and allegro_glsl.h. Then 
you had to switch based on the shader platform so you could use the 
appropriate functions. I really didn't see a point behind making it this 
annoying to use the default shaders, so how about this API instead:

No more allegro_glsl.h and allegro_hlsl.h, instead these two functions 
are added to shader.h:

const char* al_get_default_vertex_shader(ALLEGRO_SHADER_PLATFORM platform);
const char* al_get_default_pixel_shader(ALLEGRO_SHADER_PLATFORM platform);

The relevant code is then simplified from this:

#include <allegro5/allegro_glsl.h>
#include <allegro5/allegro_hlsl.h>

...

al_attach_shader_source(shader, ALLEGRO_VERTEX_SHADER, 
al_get_shader_platform(shader) == ALLEGRO_SHADER_GLSL ? 
al_get_default_glsl_vertex_shader() : al_get_default_hlsl_vertex_shader());

al_attach_shader_source(shader, ALLEGRO_PIXEL_SHADER, 
al_get_shader_platform(shader) == ALLEGRO_SHADER_GLSL ? 
al_get_default_glsl_pixel_shader() : al_get_default_hlsl_pixel_shader());

to this:

al_attach_shader_source(shader, ALLEGRO_VERTEX_SHADER, 
al_get_default_vertex_shader(al_get_shader_platform(shader)));
al_attach_shader_source(shader, ALLEGRO_PIXEL_SHADER, 
al_get_default_pixel_shader(al_get_shader_platform(shader)));

This is not including the pre-processor macros possibly involved in this 
(I looked into modifying the test driver to use shaders and I just 
balked at the mess the current API would require to support all 
configurations).

I think this is a worthwhile change as even if the usual use case 
includes people using their own shaders. Much of the time the vertex 
shader is unchanged and the main effect is obtained through the pixel 
shader only.

Also... if I'm changing this, any issues with the name of that function? 
No confusion with it actually returning the shader source vs the shader 
itself? I don't know how likely a newbie user using C would try to 
assign the output of those functions directly to ALLEGRO_SHADER*...

-SL