Added GLES2-compatible shader

[JapanYoshi]

I tried using the shader in my GLES2 project, but it contained a function it didn't support (texelFetch), so I coded a simple version of the shader on my own. It's slightly inaccurate, but it should be enough for most purposes.

• It should be compatible with all the default LUT images.
• It uses texture() instead of texelFetch().
• It doesn't do gamma correction; everything is linearly interpolated.
• It doesn't touch the alpha channel at all.

[JapanYoshi]

@thiagoamendola Hello?

[thiagoamendola]

@JapanYoshi Sorry for the late answer. According to #9, this feature was recently added to Godot 4. Does the new native implementation solve your needs or do you still see the need for your GLES2 PR?

[JapanYoshi]

There is still a need for Godot 3 to have a GLES2-compatible shader.