Support editing shaders in GLES MEC traces.
Support editing shaders in GLES MEC traces.

Adds support for editing shaders that were created before the MEC capture
started and thus have no `glShaderSource` call in the trace.

google/gapid,Qining/gapid,google/gapid,pmuetschard/gapid,Qining/gapid,pmuetschard/gapid,Qining/gapid,google/gapid,google/agi,Qining/gapid,google/gapid,google/agi,google/agi,pmuetschard/gapid,pmuetschard/gapid,Qining/gapid,google/gapid,google/agi,google/agi,google/gapid,pmuetschard/gapid,Qining/gapid,pmuetschard/gapid,pmuetschard/gapid,google/agi,google/gapid,google/agi,google/gapid,pmuetschard/gapid,Qining/gapid,Qining/gapid,google/agi