Bug 801: Refine commit 9a15aad0e5388a4b927e44d3d2ce136f32474bc2 cache TextureSequence's fragment shader hash-code

Adding TextureSequence.getTextureFragmentShaderHashCode() allowing to use a cached hash-code (performance, interface usability).

Implemented in GLMediaPlayerImpl and ImageSequence.
GLMediaPlayerImpl: Change access of most fields to private for clarity and API stability