Pre-release

Virtual controller image format update amd misc tweaks

* Imagine: Use pre-multiplied alpha in libpng PixmapReader by default to match other PixmapReader implementations
* Imagine: Draw UI graphics with pre-multiplied alpha to fix edge filtering artifacts when upscaling
* Imagine: Update bundled libpng to 1.6.39 and clean out old build configurations
* EmuFramework: Specify definitions of virtual controller images per-app for greater flexibility
* EmuFramework: Replace EmuSystem::mapVControllerButton() function with EmuApp::vControllerAssetDesc() using newer asset system and remove unneeded static variables
* EmuFramework: Draw virtual controller graphics with pre-multiplied alpha to fix edge filtering artifacts when upscaling
* EmuFramework: Brighten image when setting video position
* Update virtual controller overlay images with consistent padding and additional buttons