Fix transparency for FSprites

[BioRyajenka]

Honestly I am not quite sure what this line does, but without it transparency work as expected.
Fixes #965

[soywiz]

@BioRyajenka

korge/korge/src/commonMain/kotlin/com/soywiz/korge/render/BatchBuilder2D.kt

Lines 744 to 751 in 3bd7481

	fun DO_OUTPUT_FROM(builder: Program.Builder, out: Operand, u_OutputPre: Operand = BatchBuilder2D.u_OutputPre) {
	builder.apply {
	// We come from premultiplied, but output wants straight
	IF(u_OutputPre.not()) {
	SET(out["rgb"], out["rgb"] / out["a"])
	}
	}
	}

Basically, input and output of shaders before supported input and outputs with premultiplied & straight alphas.
Now shaders are expected to use colors with premultiplied alpha always. The thing is that when rendering to the final output buffer, it was with straight alpha, though in most cases final render buffer alpha will be 1.0 always. I wanted to use an intermediate texture to simplify shaders and always use premultiplied for input and output, but that work was not completed

[soywiz]

@BioRyajenka thanks! I have done the full cleanup now in #1162 . I have also updated tests with ./gradlew jvmTestFix. Please check with your code if the premultiplied alpha issue is fixed with that PR too.