[UXTHEME] Allow PNG images in visual styles (#5540)

This is prerequisite to supporting Vista+ iteration of the msstyles format,
and also for implementing GetThemeStream() later.

Visual styles compi

[UXTHEME] Allow PNG images in visual styles (#5540)

This is prerequisite to supporting Vista+ iteration of the msstyles format,
and also for implementing GetThemeStream() later.

Visual styles compiled for Vista+ can now use PNG images in the IMAGE group.
This does not affect the ability to use BMP images, they can be mixed now.

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