Don't check object's security when duplicating a handle of the same or 
lower access rights. Based on a patch by Vitaliy Margolen.

(Reported by Huw Davies.)

The (x,y) should be relative to the XImage, not the drawable.  The (xOrg,yOrg)
is what should be relative to the drawable.  This fixes a crash when XGetPixel
tried to access memory outside the XImage's buffer.

UpdateSurface has to modify one gl texture unit to bind the texture to
load.  To restore the correct properties for drawing gl texture unit 0
is activated and dirtified.

Fix the use of the scissor test in Clear. The rectangle is only set if
a clearing rectangle is used, otherwise the scissor test is disabled
(the pixel ownsership test should take care of the window
boarders). To get the scissor test back to the value the app set up
Clear dirtifies the state(s).

Test how shader constant limits are checked in Set*ShaderConstantF.
The vertex shader tests it based on the limits reported in the caps,
the pixel shader tests tries to find the limit manually because I
could not find a cap structure member specifying the pixel shader
constant limit.

Set*ShaderConstantF returns an error as soon as start or start + count
are bigger than the constant limit.