EmuGL: refinements to GLESv1 image_external

* EGLImageTargetRenderbufferStorageOES was incorrectly accepting
  TEXTURE_EXTERNAL_OES as a target. Revert that; the host GL will
  correctly reject it with INVALID_ENUM.

* Handle the REQUIRED_TEXTURE_IMAGE_UNITS_OES texparameter query.

* Validate texture parameters set on TEXTURE_EXTERNAL textures;
  otherwise invalid parameters would work on the emulator but not on a
  real device.

Change-Id: I49a088608d58a9822f33e5916bd354eee3709127

tools/emulator/opengl/system/GLESv1/gl.cpp

@@ -73,13 +73,9 @@ void glEGLImageTargetRenderbufferStorageOES(void *self, GLenum target, GLeglImag
         return;
     }
 
-    GET_CONTEXT;
     DEFINE_AND_VALIDATE_HOST_CONNECTION();
-
-    ctx->override2DTextureTarget(target);
     rcEnc->rcBindRenderbuffer(rcEnc,
             ((cb_handle_t *)(native_buffer->handle))->hostHandle);
-    ctx->restore2DTextureTarget();
 
     return;
 }