Added screenshot after glDraw* option to GLES2 Debugger

Also added timing mode option using utils/Timers.h.
Factored out common code to reduce size.
Improved Protobuf message.
Uploads data from glBufferData and glBufferSubData.

Change-Id: Iaae5e706235d942df81c7eada7223fb0b0583911
Signed-off-by: David Li <davidxli@google.com>