Replace the not-enough-precision 32 bit to float impl

keep the proc though to encapsulate it but uses the older
logic of rounding down to 3.13 first

mandel.s

@@ -261,28 +261,6 @@ fixed3_13_as_float: ; float48
     .byte $00
     .byte $00
 
-u65536_as_float: ; float48
-    ; 1 << 16
-    ; 65536
-    ; 06 55 36 . 00 00
-    .byte 66 ; exponent/sign - +2 bytes
-    .byte $06
-    .byte $55
-    .byte $36
-    .byte $00
-    .byte $00
-
-fixed6_26_as_float: ; float48
-    ; 1 << 26
-    ; 67108864
-    ; 67 10 88 64 . 00
-    .byte 67 ; exponent/sign - +3 bytes
-    .byte $67
-    .byte $10
-    .byte $88
-    .byte $64
-    .byte $00
-
 sec_per_frame: ; float48 00 . 01 66 66 66 67
     .byte 63  ; exponent/sign - -1 bytes
     .byte $01 ; BCD digits
@@ -1016,63 +994,14 @@ common:
 
 .endproc
 
+; rounds to 16-bit first!
 ; input in FR0, 32 bits signed 6.26 fixed
 ; output in FR0, Atari float
 ; clobbers a, x, y, FR0, FR1
 .proc fixed6_26_to_float
-    ; check sign bit! conversion routine is for unsigned
-    lda FR0 + 3
-    and #$80
-    sta temp
-
-    beq positive
-    neg32 FR0
-positive:
-
-    ; save low word
-    lda FR0
-    pha
-    lda FR0 + 1
-    pha
-
-    ; convert high word
-    sta FR0 + 2
-    sta FR1
-    lda FR0 + 3
-    sta FR0 + 1
-    jsr IFP
-
-    lda temp
-    beq positive2
-    ; set float sign bit
-    lda FR0
-    ora #$80
-    sta FR0
-positive2:
-
-    ; high word to FR1
-    ldx #.lobyte(u65536_as_float)
-    ldy #.hibyte(u65536_as_float)
-    jsr FLD1R
-    jsr FMUL
-    jsr FMOVE
-
-    ; convert low word
-    pla
-    lda temp + 1
-    pla
-    lda temp
-    jsr IFP
-
-    ; combine
-    jsr FADD
-
-    ; scale
-    ldx #.lobyte(fixed6_26_as_float)
-    ldy #.hibyte(fixed6_26_as_float)
-    jsr FLD1R
-    jsr FDIV
-
+    shift_round_16 FR0, 3
+    copy16 FR0, FR0 + 2
+    jsr fixed3_13_to_float
     rts
 .endproc