Enabled proper H264-enoding in ScrerenCapture sample

[Wu3r5l1n]

Hi and thanks for your great effort in putting this together. When running the ScreenCapture example, I noticed that the resulting file sizes were quite large. This was caused by the encoder only generating I-Frames, so no temporal correlation was used for compression. In the end it was just a configuration issue, but it caused me some time to figure it out. Opening this PR because I think this might be useful for others.

Changes:

• Most notably, set initparams.EnablePTD = 1 to enable R- and B-frames. Otherwise, only IDR-frames will be generated
• Added encodeConfig to allow user to set a desired bitrate and mode
• Changed open-mode of output file to "Create" to write a new file each time instead of writing over an existing file.

[jlennox]

Thanks for the compliment and thanks for up streaming your fix! I'm glad this project is finding use by someone.