Bugfix textureformat, only set when values are defined

Bugfix for setting a texture, only use format and type if used.
Cleanup of destroy method

Simplify naming and usage of destroy method for pooled object and blit
object.
J2SE property for setting DPI

Make it possible to force specific DPI on J2SE platforms. Needed when
the system does not have any reliable metrics, for instance on desktop.
Do not throw exception at error when clearing buffer.