more Kernal reorganization

c64/kernal/screen/e50a.interop

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+The original reason to move PLOT routine into its original position was to provide support for unofficial $E50C address:
+
+- it is well known: https://www.lemon64.com/forum/viewtopic.php?t=3296&amp%3Bstart=15
+                    https://sys64738.org/2019/05/c64-bedtime-coding-eng-printing-08/
+- 'Duotris' game calls $E50C during startup screen initialization

c64/kernal/screen/e50a.plot.s

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+
+//
+// Official Kernal routine, described in:
+//
+// - [RG64] C64 Programmer's Reference Guide   - page 290
+// - [CM64] Compute's Mapping the Commodore 64 - page 215
+//
+// CPU registers that has to be preserved (see [RG64]): none
+//
+
+PLOT:
+	bcs plot_get
+
+	// Note: the 'set' part has to start from $E50C, as this is a known address
+	// - https://www.lemon64.com/forum/viewtopic.php?t=3296&amp%3Bstart=15
+	// - https://sys64738.org/2019/05/c64-bedtime-coding-eng-printing-08/
+	// - 'Duotris' game calls $E50C during startup screen initialization
+
+plot_set:
+
+	sty PNTR
+	stx TBLX
+
+	jsr screen_calculate_line_pointer
+
+	// FALLTROUGH to save one byte on RTS
+
+plot_get:
+
+	ldy PNTR
+	ldx TBLX
+	rts