Clean up debug string functions in OS_HLE.

 * Use GetString instead of GetPointer.
 * Use C++ strings instead of char* pointer math.
GLX: try to get an OpenGL 3.3 core context
Only use bounds check for DVDLowUnencryptedRead
Check if there is a disc for all disc reads
Mark cover as open when no disc is inserted
Always read 0x20 bytes of disc ID
FramebufferManager: Copy all EFB layers to the XFB framebuffer.
OGL: Support stereoscopic XFB blit to screen.
FramebufferManagerBase: Only allocate one layer for Real XFB.
GeometryShaderCache: Cosmetics.
FramebufferManager: Copy all EFB layers to the XFB.
D3D: Support stereoscopic XFB blit to screen.
VideoConfig: Turn off stereoscopy when Real XFB is enabled.
D3D: Remove obsolete Real XFB codepath.
FramebufferManager: Return framebuffer target size in GetTargetSize().
FramebufferManagerBase: Remove obsolete parameters.
Renderer: Use old method of calculating the source rectangle.
XFBEncoder: Sample the first layer of the resolved EFB texture.

Using the multisampled EFB texture is invalid, as the XFB encoder does not have a multisampling shader.
Merge pull request #1740 from Armada651/stereo-xfb

Support stereoscopy with Virtual XFB
Merge pull request #1733 from degasus/glx

GLX: try to get an OpenGL 3.3 core context