Implemented global const array to shaders

[Chaosus]

This PR allowed const array to be declared outside of function.

As usual, when this will be merged I've add that to Vulkan too.

[clayjohn]

Have you tested this on GLES2 devices that only support up to GL 2.1 or GL ES 2.0? I don't think constant arrays are possible in GLES2 devices.

Edit: I just looked at the GL ES 2.0 GLSL spec and it says that it is implementation dependant. It does not require that constant array initialization be allowed. But implementers can allow it if they want.

I ran into troubles with it in my GLES2 Glow PR as it caused certain low end devices to fail to compile the shaders.

[Chaosus]

Yeah :( It may not be supported on all devices... I want to hear a @reduz decision about this.

[reduz]

We may only allow it for static indexing on GLES2, but thats more work on the compiler on our side to validate..

[Chaosus]

I've removed it from GLES2 backend and added a check to prevent its usage on that platform...

[Chaosus]

Note: I rebase it again when Vulkan will be merged to the master

[aaronfranke]

Vulkan has been merged into master, this can be rebased now.

[Chaosus]

Rebased again...