egl: Simplify queries for EGL_RENDER_BUFFER

There exist *two* queryable EGL_RENDER_BUFFER states in EGL:
eglQuerySurface(EGL_RENDER_BUFFER) and
eglQueryContext(EGL_RENDER_BUFFER).

These changes eliminate potentially very fragile code in the upcoming
EGL_KHR_mutable_render_buffer implementation.

* eglQuerySurface(EGL_RENDER_BUFFER)

  The implementation of eglQuerySurface(EGL_RENDER_BUFFER) contained
  abstruse logic which required comprehending the specification
  complexities of how the two EGL_RENDER_BUFFER states interact.  The
  function sometimes returned _EGLContext::WindowRenderBuffer, sometimes
  _EGLSurface::RenderBuffer. Why? The function tried to encode the
  actual logic from the EGL spec. When did the function return which
  variable? Go study the EGL spec, hope you understand it, then hope
  Mesa mutated the EGL_RENDER_BUFFER state in all the correct places.
  Have fun.

  To simplify eglQuerySurface(EGL_RENDER_BUFFER), and to improve
  confidence in its correctness, flatten its indirect logic. For pixmap
  and pbuffer surfaces, simply return a hard-coded literal value, as the
  spec suggests. For window surfaces, simply return
  _EGLSurface::RequestedRenderBuffer.  Nothing difficult here.

* eglQueryContext(EGL_RENDER_BUFFER)

  The implementation of this suffered from the same issues as
  eglQuerySurface, and the solution is the same.  confidence in its
  correctness, flatten its indirect logic. For pixmap and pbuffer
  surfaces, simply return a hard-coded literal value, as the spec
  suggests. For window surfaces, simply return
  _EGLSurface::ActiveRenderBuffer.

Reviewed-by: Tapani Pälli <[email protected]>

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