[AD] More default shaders

[SiegeLord <slabode@xxxxxxxxxx>]

I think the shaders are a very cool concept but are hard for newbies to 
start with. I think the shader addon should provide a whole slew of 
default shaders for nice effects with an easy API that doesn't leak the 
implementation details (like the current al_get_glsl_*_shader do). 
Something that would recapitulate the effects you could get in A4 using 
the blenders (see 
http://alleg.sourceforge.net/stabledocs/en/alleg020.html for a list), 
but possibly add a number of other useful ones, like blurring or glass.

One simple API idea would be to just do this:

ALLEGRO_SHADER* al_create_default_shader();
ALLEGRO_SHADER* al_create_light_shader();
//etc

After being created, the user would interact with the shader the usual 
way (the various variables, such as colors, will be set using the 
uniform setting syntax we already have).

It might be interesting to simplify that even further and mimic the A4 
API more accurately (ignore the names, no thought was put into them):

ALLEGRO_EFFECT_STATE* al_create_effect_state();

al_use_default_effect(ALLEGRO_EFFECT_STATE* state);
al_use_light_effect(ALLEGRO_EFFECT_STATE* state, ALLEGRO_COLOR light);

Obviously this is inflexible, but it is exactly designed for ease of 
use by beginners. The old API is unaffected by this proposed change 
(even the al_get_default_glsl_*_shader have their use, just not this 
use).

-SL