extract the artist grid scroll estimate and test it

The index to pixel arithmetic was inline and fiddly (it was missing the row gap), so pull it
out as estimateScrollPx and pin the 2-column cases. ROW_GAP is shared with the grid spacing.

app/src/test/kotlin/com/subtracks/ui/library/EstimateScrollPxTest.kt

@@ -0,0 +1,30 @@
+package com.subtracks.ui.library
+
+import org.junit.Assert.assertEquals
+import org.junit.Test
+
+class EstimateScrollPxTest {
+    private val image = 432f
+    private val album = 200f
+    private val gap = 16f
+
+    @Test
+    fun theHeroUsesTheRawOffset() {
+        assertEquals(50f, estimateScrollPx(0, 50f, image, album, gap), 0.01f)
+    }
+
+    @Test
+    fun theFirstAlbumRowAddsTheHeroAndOneGap() {
+        assertEquals(image + gap + 10f, estimateScrollPx(1, 10f, image, album, gap), 0.01f)
+    }
+
+    @Test
+    fun theSecondCellOfARowDoesNotAdvance() {
+        assertEquals(estimateScrollPx(1, 10f, image, album, gap), estimateScrollPx(2, 10f, image, album, gap), 0.01f)
+    }
+
+    @Test
+    fun theNextRowAddsACellAndAGap() {
+        assertEquals(image + gap + (album + gap) + 10f, estimateScrollPx(3, 10f, image, album, gap), 0.01f)
+    }
+}