Improve ETC1 support

This pull request : 
- Add DDS with ETC1 support inside the WMS client (example included, needs to have dds with ETC1 support server side)
- Add the capability to compress to DDS with ETC1 using the RenderscriptCompressor lib : Functionnality  activated on BMNG and Landsat layer, to activate it in the xml :  <ImageFormat>image/jpeg</ImageFormat> <FormatSuffix>.dds</FormatSuffix> png is not yet supported
- By default JPEG are now decoded to RGB565 bitmap/texture instead of RGBA8888
- Include some code to check if ETCx/DXTx are available at startup (but not yet used)