Implement mix color shader.

[andy840119]

See mixed texture deleted before, not really sure will be implemented by shader 🤔
https://github.com/karaoke-dev/osu-framework-font/blob/2021.0708.0/osu.Framework.Font/Graphics/Sprites/ILyricTexture.cs

[andy840119]

Update:
Should be able to accept array in GetUniform<T> (See SetupUniforms in Shader.cs)

[andy840119]

Or calculate the sample shader by CPU.
But it might cause lots of delay if running in graphic thread.

[andy840119]

https://git.buttonbashed.co.nz/shadoh/Explore/src/master/Exploration/GLObjects/ShaderProgram.cs
Not really sure that will unitorm array in shader will become to each params.

e.g: point_1, point_2 ...

[andy840119]

tested.
seems osu-framework only able to assign first variable in array...

[andy840119]

https://gamedev.stackexchange.com/questions/79376/slow-uniform-array-in-shader-causing-fps-drop
Also, found an issue that will cause performance while reading data from array.
So the tricky way might be place passing data into sample2D