mesa: Make gl_renderbuffers backed by EGL images use FinishRenderTexture.

mesa: Make gl_renderbuffers backed by EGL images use FinishRenderTexture.

This is the opportunity that radeon and intel drivers rely on for flushing
render targets that may get reused as textures.  Before EGL, that only
happened for GL_TEXTURE attachments.

Fixes piglits:
KHR_gl_renderbuffer_image/renderbuffer-texture
OES_EGL_image/renderbuffer-texture

NOTE: This is a candidate for the 9.1 branch.
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>