Fix incorrect cropping in AlphaBlending element

[t-mullen]

What is the current behavior you want to change?

When the relative X/Y is greater than zero but less than the relative width/height, the video is cropped incorrectly.
Fixes Kurento/bugtracker#63.

ie) Given setPortProperties(0.1, 0.1, 0, 0.5, 0.5, port) results in the video being positioned at 0.1, 0.1 but only a small slice of the video is showing, the rest is cropped.

What is the new behavior provided by this change?

Removing this cropping produces the expected behaviour with correct relative X, Y, width, height.

ie) Given setPortProperties(0.1, 0.1, 0, 0.5, 0.5, port) results in the video being positioned at 0.1, 0.1 with the entire video showing, correctly scaled to 50% of the master port size.

How has this been tested?

Tested in Ubuntu 18 with both latest KMS source and as a module in KMS 6.14.1 installed through apt.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature / enhancement (non-breaking change which improves the project)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• My change requires a change to the documentation
• My change requires a change in other repository

Checklist

• I have read the Contribution Guidelines
• I have added an explanation of what the changes do and why they should be included
• I have written new tests for the changes, as applicable, and have successfully run them locally

[jenkinskurento]

Hi there, thanks for your Pull Request!

A Kurento member needs to verify that this patch is reasonable to test. In case it is, they should write a comment with the phrase test this please. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by Kurento members will still work. Regular contributors can be whitelisted to skip this step.