Mobile NeRF general fixes

[RodrigoHolztrattner-QuIC]

Description

• Fix combo mode not properly using the right MLP weights.
• Fix 'image_layout_transition()' at 'setup_attachment()' throwing an error when using VK_IMAGE_LAYOUT_GENERAL.
• Initialize VkAttachmentDescriptions to fix validation issues.
• Use vkb::sg::Image::Other instead of vkb::sg::Image::Color so textures are properly loaded as linear color space instead sRGB.
• Other clean up and modification:
  1. Remove weights_buffer_ref & uniform_buffer_ref, which are raw ptrs pointing to unique ptrs in vectors. This is unsafe, as everything will mess up when vector gets reallocated. Use indices as references instead.
  2. No need to update weights buffers for every frame, only upon their creation.
  3. No need to have samplers for G-buffer attachments because we directly use subpass load, removed.
  4. No need for mlp.comp shader, removed.
  5. No need for aligned_size() function, removed.
  6. No need to include "stb_image.h" in mobile_nerf.cpp.
  7. Remove debug code in shaders.

General Checklist:

Please ensure the following points are checked:

• My code follows the coding style
• I have reviewed file licenses
• I have commented any added functions (in line with Doxygen)
• I have commented any code that could be hard to understand
• My changes do not add any new compiler warnings
• My changes do not add any new validation layer errors or warnings
• I have used existing framework/helper functions where possible
• My changes do not add any regressions
• I have tested every sample to ensure everything runs correctly
• This PR describes the scope and expected impact of the changes I am making

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[SaschaWillems]

Code changes look fine to me. Works on my RTX 4070 on WIn11.