Examples: Fix Firefox Error in "webgl_interactive_cubes_gpu"

[Mugen87]

This PR fixes an error in webgl_interactive_cubes_gpu logged by Firefox 45.0.1.

Error: WebGL: readPixels: Out-of-bounds reads with readPixels are deprecated, and may be slow.

The problem is that the example can generate read requests with readRenderTargetPixels that contain out-of-bounds pixels (see FF source code for more information). To avoid this problem, an additional if statement prevents read request with mouse coordinates less or equal zero.

A more strict implementation could be done in readRenderTargetPixels. The method could only allow read request with x and y values within the range of [1, renderTarget.width] and [1, renderTarget.height].

[mrdoob]

A more strict implementation could be done in readRenderTargetPixels. The method could only allow read request with x and y values within the range of [1, width] and [1, height].

That sounds like a better fix!

[Mugen87]

Done!

[mrdoob]

Thanks!