Default Tensor type fix

[ili3p]

There is a bug if you set the default tensor type to CudaTensor. For example:

 local display   = require 'display'
torch.setdefaulttensortype('torch.CudaTensor')

-- then call
display.image(torch.randn(16, 3, 32, 32)

The display.image would call image.compressJPG(img) which in turns calls image.clampImage(..). There, these two lines cause problem:
init.lua165: from image package

local a = torch.Tensor():resize(tensor:size()):copy(tensor)
a.image.saturate(a) -- bound btwn 0 and 1

Here a is a default type tensor and if that is set to CudaTensor it won't have the field image.

This PR is fixing the bug by setting the default tensor type to DoubleTensor and then restoring to the original state.

[ili3p]

I have also changed the image button to a bolder arrow and add the image button only to ImagePanes.