GLSL ES 3.x support?

[JohnnyonFlame]

I'm having issues with MRT on MojoShader (GLES backend) due to the shader version used (glsl es 1.x - #version 100)

I threw this together as an initial test: https://github.com/JohnnyonFlame/mojoshader/tree/fna_es3 And it seems to work for a few of the games I've tested such as Anodyne (which is affected by the missing MRT on GLSL ES 1.x), TMNT:SR and Salt and Sanctuary. But I'm not 100% sold on the correctness of this patch to actually push it as a PR. Pushed on #65

Is this patch interesting at all? Anything I can do to properly test/any issues you can foresee here?