emulator: opengl: Add custom_write optimization to encoder.

This patch allows an auto-generated GLES encoder function to write
'isLarge' buffers with a custom writer, instead of calling stream->readFully()

This is intended to allow writing pixel or vertex data that is stored
with a specific stride.

Another patch will introduce the corresponding changes to the .attrib files

Change-Id: I6ca86b968cd3f4db91676bc485ee1e84419e50e0
5 files changed