blitbuffer: RGB32 support for rounded corners

[hius07]

To be used in colored buttons.

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This change is 

[NiLuJe]

I'd obviously prefer that with the C implementation in, too ;).

(At a quick glance, it wouldn't look too gnarly to implement, just a bunch of code duplication).

[hius07]

Looks like it's already there but the color parsing should be fixed:

koreader-base/blitbuffer.c

Lines 3151 to 3152 in b677605

	} else if (bb_type == TYPE_BBRGB32) {
	const ColorRGB32 color = { .r = c, .g = c, .b = c, .alpha = 0xFF };

Out of my abilities unfortunately.

[NiLuJe]

Yeah, it already supports every target BB, which is why I mentioned it should be fairly easy (basically the same idea as fill_rect vs. fill_rect_RGB32) ;).

I miiiiight have some time during winter break, but who knows really ;p.

[hius07]

Lua version works fine on Kindle, frontend PR is ready.