Transparency for custom shader materials not working

[srmainwaring]

Environment

• OS Version: macOS Big Sur 11.6.2
• Source build, main branch, commit ff16689f
• If this is a GUI or sensor rendering bug, describe your GPU and rendering system
  • Rendering plugin: [ogre2].
  • Generally, mention all circumstances that might affect rendering capabilities:
    • [x] running on real hardware
    • [x] GPU is concurrently used for other tasks
      • [x] desktop acceleration
  • Rendering system info:

system_profiler SPDisplaysDataType
2022-05-26 12:09:53.871 system_profiler[93720:261323859] Device PreExisted [00000001000009b6] AMD Radeon Pro W5700X
Graphics/Displays:

    AMD Radeon Pro W5700X:

      Chipset Model: AMD Radeon Pro W5700X
      Type: GPU
      Bus: PCIe
      Slot: Slot-1
      PCIe Lane Width: x16
      VRAM (Total): 16 GB
      Vendor: AMD (0x1002)
      Device ID: 0x7310
      Revision ID: 0x0000
      ROM Revision: 113-D1860W-213
      VBIOS Version: 113-D186A2XT-019
      Option ROM Version: 113-D186A2XT-019
      EFI Driver Version: 01.01.200
      Metal Family: Supported, Metal GPUFamily macOS 2
      Displays:
        Cinema HD:
          Display Type: LCD
          Resolution: 2560 x 1600
          UI Looks like: 2560 x 1600
          Framebuffer Depth: 30-Bit Color (ARGB2101010)
          Display Serial Number: CY628129UG1
          Main Display: Yes
          Mirror: Off
          Online: Yes
          Rotation: Supported
          Automatically Adjust Brightness: Yes
          Connection Type: DVI or HDMI
          Adapter Firmware Version: 2.21

Description

Expected behaviour:

Objects should be partially visible beneath/behind objects rendered with custom shader materials when that object has the <transparency> SDF element set, or the fragment shader for that object has an final alpha value < 1.0

Actual behaviour:

The object using the custom shader is opaque.

Steps to reproduce

1. Run the OSRF waves example: https://app.gazebosim.org/OpenRobotics/fuel/models/Waves
2. Add another partially submerged object to the world

Output

The submerged portion of the red sphere is completely obscured by the sea surface.

Edit @iche033 btw I'm happy to help put a PR together for this, or help with testing, but could do with a few pointers on where to look. I can see where the Hlms::BlendBlock is being set, but don't quite follow how the properties are propagated to the HllmLowLevel materials used for backwards compatibility with the Ogre(1) material system.

[azeey]

Related to https://github.com/gazebosim/gz-rendering/issues/846