After landing flutter/engine#50416 I was able to get an app to run with the embedder using the Impeller GLES backend on desktop Linux.
But it required some additional changes for desktop OpenGL.

• Current versions of desktop OpenGL do not support the GL_ALPHA texture format in glTexImage2D.
  • The glyph atlas needs to instead use GL_RED.
• BufferBindingsGLES::BindVertexAttributes must bind to a vertex array object before calling glVertexAttribPointer

Also, the Linux/OpenGL embedder is no longer rendering anything on screen after flutter/engine#50539.

flutter/engine#50539 was originally specific to the Vulkan back end. Does the OpenGL backend need to do something to match Vulkan's behavior?

@jonahwilliams @chinmaygarde