etna: move etna_convert_r8g8b8_to_b8g8r8x8

This function is only used in the demos, move it out of the driver.
MESA has better functions for conversion between pixel formats in any
case...
etna: render to surfaces without Tile Status

Support rendering to surfaces that do not have a Tile Status buffer.
This makes it possible to render to textures, for example.