Add glReadPixels with buffer-offset argument

- For reading into PBOs on GLES3, add an overloaded version of
  glReadPixels that takes an offset argument.

Bug 18878609

Change-Id: Ide9beca7b241a9f8a016e8ae17082bd0ba21246a
diff --git a/opengl/tools/glgen/specs/gles11/GLES30.spec b/opengl/tools/glgen/specs/gles11/GLES30.spec
index a426eb0..f190fc0 100644
--- a/opengl/tools/glgen/specs/gles11/GLES30.spec
+++ b/opengl/tools/glgen/specs/gles11/GLES30.spec
@@ -108,3 +108,4 @@
 void glTexStorage2D ( GLenum target, GLsizei levels, GLenum internalformat, GLsizei width, GLsizei height )
 void glTexStorage3D ( GLenum target, GLsizei levels, GLenum internalformat, GLsizei width, GLsizei height, GLsizei depth )
 void glGetInternalformativ ( GLenum target, GLenum internalformat, GLenum pname, GLsizei bufSize, GLint *params )
+void glReadPixels ( GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLint offset )