ControllerRenderingEngine: Disable BGRA when targeting Wasm

[fwcd]

This seems to be be unavailable when targeting WebAssembly/Emscripten, even though we already require WebGL 2.0, which is a subset of OpenGL ES 3.0:

/home/runner/work/m1xxx/m1xxx/mixxx/src/controllers/rendering/controllerrenderingengine.cpp:77:55: error: use of undeclared identifier 'GL_BGRA'
   77 |         m_GLDataFormat = m_screenInfo.reversedColor ? GL_BGRA : GL_RGBA;
      |                                                       ^
1 error generated.

Since other OpenGL ES implementations (e.g. iOS) do implement this extension, I have scoped it to non-Emscripten-platforms. Otherwise this follows the approach taken in #13382.

[JoergAtGithub]

LGTM! Thank you!