Add time uniform to glsl shaders

[resloved]

Added an updating uniform that can be used to animate GLSL shaders. As discussed in #295

After looking at the different options available already in picom I was able to have a pretty limited changeset to get the effect I wanted. If you want to test it you can use this shader I wrote:

uniform float opacity;
uniform bool invert_color;
uniform sampler2D tex;
uniform float time;

float amt = 2000.0;

void main() {
        float pct = mod(time, amt)/amt;
        vec2 pos = gl_TexCoord[0].st;
        vec4 c = texture2D(tex, pos);
        float a = pos.x + pos.y;
        if (a < pct * 4.0 && a > pct * 4.0 - 0.5 * pct)
           c *= vec4(2, 2, 2, 1);
        else if (a < pct * 4.0 - 0.8 * pct && a > pct * 3.0)
           c *= vec4(2, 2, 2, 1);
        if (invert_color)
           c = vec4(vec3(c.a, c.a, c.a) - vec3(c), c.a);
        c *= opacity;
        gl_FragColor = c;
}

Then run picom with these changes and the following flags (This assumes you have the previous shader saved to ~/shine.glsl):

picom --sw-opti --benchmark 100000 --no-use-damage --backend glx --glx-fshader-win "$(cat ~/shine.glsl)"

It should look something like this:

https://streamable.com/e0x7r

Some issues:

• Using benchmark is obviously really hacky. Changes from the shader won't queue a redraw, so we can't wait on draw_idle to be enabled. Either these changes would need to affect draw_idle or there would need to be a nicer way to ignore it.
• Similarly I have to ignore damage as changes from the shader are ignored. Not being idle can really tear up performance and finding a solution is probably necessary. Luckily --sw-opti cuts the cost significantly for me, but it doesn't fix everything.

I really think with a few more options you could do some pretty powerful stuff, but it might be out of the scope of this pull request. Some basic stuff that comes to mind:

• Timer since window was created
• Timer since window was focused/last focused
• Window dimensions
• And the numerous other values that a window can have (class, name, etc.)

[yshui]

Looks mostly fine, can you fix the build failures?

[yshui]

What do you think is better for animation, time or frame count? Should we have both?

[resloved]

Traditionally using the time is better as its consistent across different levels of performance and refresh rate. Ideally it lets you write one shader that works across platforms.

[yshui]

Thanks, I see no problem of this.

It would be nice if you can also implement this for the new glx backend.

[sabrehagen]

These days the glsl code in the first post of this PR gets the following error:

[ 08/23/2023 12:55:32.616 gl_win_shader_from_stringv ERROR ] Failed to create GLSL program.
[ 08/23/2023 12:55:32.616 initialize_backend WARN ] Failed to create shader for shader file /home/jackson/.config/picom/shaders/shine.glsl, this shader will not be used

config:

backend = "glx"
window-shader-fg-rule = [
  "shine.glsl:class_g = 'XTerm'"
];

Is there anything else I need to do? Or is the code the issue, and no longer supported?