gsdx: Add some comment on GSTextureCache + schematics

Note: docs/TextureCache.odg was created with libreoffice. Not perfect but give a nice overview
gsdx-ogl: reduce complexity of clear texture

Null is equivalent to a clear to 0.

Note: Code is not yet used because both stencil and depth are cleared.
Future note: stencil can potentially be replaced by load_store_image
gsdx-ogl: reduce pbo complexity

Copy the full line into the pbo. Dma will only take GL_UNPACK_ROW_LENGTH
- increase memcpy size by 2 in the pbo
+ single memcpy will be faster and can use sse

Enable buffer_storage extension:
* GL_CLIENT_STORAGE_BIT was required (it is the duty of TexSubImage to copy data into the GPU mem)
* Enable the extension by default
gsdx-ogl: only call PixelStorei when parameters are updated

It won't improve performance but it would reduce a bit the noise in gl retracer tool