Volume Rendering GLSL warning

[jzy3d]

When running BasicVolumeDemo, a warning is displayed at GLSL program validation step.

It is not a blocker but it is worth mentioning it due to #220 and #221

Ubuntu 20 on Dell OptiFlex

GLSLProgram: active samplers with a different type refer to the same texture image unit. 
../src/intel/isl/isl.c:2023: FINISHME: ../src/intel/isl/isl.c:isl_surf_supports_ccs: CCS for 3D textures is disabled, but a workaround is available.

MacOS 10.12

GLSLProgram: GLSL VALIDATION: Validation Failed: Sampler error:
GLSLProgram: GLSL VALIDATION: Samplers of different types use the same texture image unit.
GLSLProgram: GLSL VALIDATION: - or -
GLSLProgram: GLSL VALIDATION: A sampler's texture unit is out of range (greater than max allowed or negative).

MacOS 11.4

GLSLProgram: GLSL VALIDATION: Validation Failed: Sampler error:
GLSLProgram: GLSL VALIDATION: Samplers of different types use the same texture image unit.
GLSLProgram: GLSL VALIDATION: - or -
GLSLProgram: GLSL VALIDATION: A sampler's texture unit is out of range (greater than max allowed or negative).

[jzy3d]

Someone suggested

I think the shader code warning comes because of an ordering issue around when the glsl code is loaded and when the textures are init'd (so the shader code is initially pointing to the default texture), but its never executed in this state so its doesnt cause any issue (except the warning).