TWP: FREESCAPE: Fix non-compliant GLSL

[chkuendig]

OpenGLES 2.0 GLSL requires:

• Control flow is limited to forward branching and to loops where the maximum number of iterations can easily be determined at compile time. (Appendix A, 4 Control Flow)
• In the fragment shader, support for indexing is only mandated for constant-index-expressions. (Appendix A, 5 Indexing of Arrays, Vectors and Matrices)

Generally this doesn't seem to be a limit enforced on most hardware, but for WebGL, this conformity seems to be enforced by the ANGLE Compiler used in all major browsers.

This PR, fixes the following two errors when launching TWP or Freescape games:

• '[]' : Index expression must be constant
• 'i' : Loop index cannot be compared with non-constant expression

I tried to figure out correct upper boundary for the new iteration limits, but would be good if somebody more knowledgeable could double check.

[neuromancer]

I just tested freescape using the new shaders and looks good. I think we need @scemino to take a look to the TWP changes and we are ready to go.

[scemino]

I read your commits, it seems good to me, but I didn't test it.