render consecutive shaders to the fbo (opengl)

[juliapaci]

fixes #4729

allows the shaders to sample each other via the fbo texture.

also, a better example would use the full screen e.g.:
"behind.glsl"

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    fragColor = vec4(fragCoord/iResolution.xy, 0.0, 1.0);
}

"infront.glsl"

void mainImage( out vec4 fragColor, in vec2 fragCoord )
{
    fragColor = texture(iChannel0, fragCoord/iResolution.xy);
}

ghostty --custom-shader=behind.glsl --custom-shader=infront.glsl

before	after