Allow mixing of static and dynamic selector values

[katamartin]

For array-based selector values, we currently assume that the values of the array are fixed and directly map the all elements of the array to generate shader variables. In practice, this means that if you want to dynamically reference elements of the array, extra logic and uniforms must be used to handle dynamic selection within the fragment shader. This is less-than-ideal for a number of reasons:

• subject to 16 variable limit for texture definitions in the fragment shader
• loading unused data onto GPU
• requires some gymnastics with additional uniforms (see example)

We should consider undoing this assumption and updating the selector prop to accommodate this type of "mixed" selection.

Example of current integration

Consider an example where multiple variables (in this case multiple measures of the same variable, prec) can be averaged over area).

in Map definition

uniforms={{measureUniform}}
selector={{
  band: ['area', 'prec_measure_1', 'prec_measure_2', 'prec_measure_3', ...],
}}

in fragment shader:

float prec;
if (measureUniform == 1.0) {
  prec = prec_measure_1;
}
if (measureUniform == 2.0) {
  prec = prec_measure_2;
}
  ...

float value = prec / area;

Potential integration

in Map definition

selector={{
  band: ['area', {prec: `prec_measure_${measurementId}`}],
}}

in fragment shader:
(no custom shader code for measure selection required, prec can be used directly)

float value = prec / area;