gr-fosphor on AMD RX 550

[Johannes Demel]

Hi all,

We have some new AMD machines with RX 550 graphics. I want to run 
gr-fosphor on them. So far I've failed to make it work.

It's an AMD system with RX 550 and Ubuntu 18.04 with amdgpu open source 
drivers. I work with GR 3.8-master (or 3.9.0.0-git) 
`4f53ac5a76e3ab05960a81905a570cd74d2708d7`

Since it was quite tricky for me to install OpenCL and make gr-fosphor 
find it, I just document my workflow really quick.

I found this website:
https://einsteinathome.org/content/quick-guide-how-install-opencl-amd-gpus-linux-kubuntu-1804-and-similar-distro

First, I went to amd.com and got the drivers for my GPU:
https://www.amd.com/en/support/graphics/radeon-500-series/radeon-rx-500-series/radeon-rx-550

Then I ran:
./amdgpu-pro-install --opencl=legacy,pal
This does only install OpenCL

Further, I had to install these packages on Ubuntu:
libfreetype6-dev
ocl-icd-opencl-dev
libglfw3-dev

Also, `clinfo` was useful because it gives you some info about your system.

So finally, I could compile and install `gr-fosphor`. It appears in GRC 
and I can plug the block (Qt version) in a flowgraph.

But if I run this flowgraph, I get the following error:
```
[!] gl_cmap shader compilation failed (cmap_simple.glsl)
[w] Color map shader 'simple' failed to load, will use fallback
[!] gl_cmap shader compile log :

[!] gl_cmap shader compilation failed (cmap_bicubic.glsl)
[w] Color map shader 'bicubic' failed to load, will use fallback
[!] gl_cmap shader compile log :

[!] gl_cmap shader compilation failed (cmap_fallback.glsl)
[!] Color map shader 'fallback' failed, aborting
gr::log :ERROR: qt_sink_c0 - Failed to initialize fosphor
```

I have no idea why and so far googling brought me to some macports 
threads that were related to Intel GPUs on MacOS.

Did someone already succeed with gr-fosphor on this hardware? Do I have 
suitable hardware? Did I fail the installation process somewhere? Do I 
need to provide additional info?

Cheers
Johannes