gsdx-ogl: GL_ARB_shading_language_420pack is now mandatory

All API/drivers support it so time to remove the fallback.
gsdx-ogl: Use 32B strides for all VBO
gsdx-ogl: merge the two vertex buffer format

* Only a single VAO
    => Format is set once
    => Only a single bind at startup
    => GSVertexBufferStateOGL is nearly useless
    => barely faster but better than nothing :)
gsdx-ogl: mark GL_EXT_shader_io_blocks as mandatory for GLES
gsdx-ogl: optimize the PS cb cache

* Don't use the stack
* Don't compare MinMax parameter (depends of others)
* Don't store not-compared parameter in the cache (HalfTexel/MinMax)

+0.3fps/46fps (well better than nothing)