transparent splash logo & fix tab widget color glitch

[zxkmm]

1. made black as transparent color (i checked the code, all the place who use this func is not impacted, harmless, previously its purple), work i promised to do in add bitmap arr to file (or reverse) converter #2248
2. same fix for the fake 3D effect for grid (reword of #2158 #2249)

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the goal of this change is to fix this situation that the splash screen has a black square if you are using not default theme.

The original fake transparent color is (255,0,255), but when i make the bg became it, the bmp always took more spaces. so if i do (1,1,1), it would also be meaningless.

I'm not familiar with bmp format but i'm guessing some of the color seems tooks more spaces than black.

there are only two places that use this func: draw default splash (not custom) and modal warning image. and if someone added more in the future, you can freely choose not to apply transparent when you call that func.

and considering in the most case, black is the background color, so i think it's more appropriate.

[zxkmm]

[gullradriel]

LGTM