Fixed Ximea cameras support
Fix two bugs related to opencl context.
1. As getDevice will implicitly call setDevice, in getContext we should not need to call it again.
2. Fix an incorrect type casting.
Add a function to query if global OpenCL context is initialized.
Fixes bug #3071.

If we have perfect matches (min_dist == 0.0), then strict comparison
fails. Making it non-strict results in treating perfect matches as
good.
Javadoc waring fix.
Fixed a bug related to video stabilization crashes with a blank video (Bug #3023)
Merge pull request #991 from asmorkalov:android_javadoc_fix
Merge pull request #981 from SeninAndrew:ximea_camera_support_fix
Merge pull request #982 from pengx17:2.4_fix_two_bugs
Merge pull request #986 from pengx17:2.4_initiated_context
Merge pull request #988 from snosov1:3071-fix
Merge pull request #993 from ivan-korolev:fix_videostab_bug_3023
Merge branch '2.4' of https://github.com/Itseez/opencv into fix_sift_bug_2892
Added regression tests for SURF/SIFT (related to #2892)