opengl: replace multi-variables by arrays in GLSL

Variables were generated on the fly with an index in the name. For
example:

    uniform sampler2D Texture0;
    uniform mat3 TexCoordsMap0;
    uniform sampler2D Texture1;
    uniform mat3 TexCoordsMap1;
    uniform sampler2D Texture2;
    uniform mat3 TexCoordsMap2;

Replace them by arrays:

    uniform sampler2D Textures[3];
    uniform mat3 TexCoordsMaps[3];

This will enable to expose a GLSL function providing a color for a
particular plane number passed as a parameter.

Signed-off-by: Alexandre Janniaux <ajanni@videolabs.io>