[KOPRajs]

@garbear I have played with the shaders again.
I wanted to try to build LibreELEC for my ARM box with the GLES shader support and I wanted to test, whether it behaves the same, as it did when we were trying it on Wayland with Joe.

1. The build itself went fine, no errors there.
2. Then I built the game.shaders.preset addon.
3. I've found out that the path to the shaders is hardcoded as "special://xbmcbinaddons/game.shaders.preset/resources/...". It cannot be used with such path when the addon is installed from .zip (or repository). So I needed to change the hardcoded path to "special://home/addons/game.shaders.preset/resources/...". Then I got the presets to load from the XML. I've also added the actual shaders to the correct folder before.
4. But when selecting a shader preset in the Video Filter menu, I get only the nearest filter. No errors in log, everything looks good, except there is no shader being applied at all.
5. I've found out that my LibreELEC build for RK3399 is actually not using the OpenGL ES renderer directly. There is another renderer called DMA, which is a special case of the OpenGL ES render. It seems that for GBM windowing this renderer is used instead of the OpenGL ES one.
6. So I ended up adding the shader support code to the RPRendererDMA as well and (to my surprise) now I've got the shaders working on my LibreELEC box (with the same bugs and limitation as with OpenGL ES).

The code is here, if it can be of any use: https://github.com/KOPRajs/xbmc/commits/video-shaders
The 2 relevant commits are:
https://github.com/KOPRajs/xbmc/commit/0...443716301f
https://github.com/KOPRajs/xbmc/commit/3...7abb8e98c7

Again, be aware, I do not fully understand the code. I've only copied a chunk of code from the OpenGL renderer to the DMA render (the same as for the OpenGL ES).