TESTBED: Add shader compatibility tests and remove incompatible shaders from Emscripten build - #6990

I noticed that many of the shaders we imported from libretro are not compatible with WebGL (and probably with OpenGL ES either). Having ~30% of the selection in the options not work is not great. This PR changes three things:

• Add a test to testbed that tests all shaders in shaders.dat as part of the graphics subset tests (can be run in non-interactive mode) and reports which shaders are incompatible.
• Removes these shaders in the dist-emscripten build target in the relevant makefile.
• Removes the shader update button in Emscripten, since it has no use there (if we want to update the shaders, we can just replace the default shader.dat file on the server) and might download incompatible shaders.