Fix background blending sometimes setting alpha to zero in 32bpp modes

Detail:
  s/Blending - blend_putdata_32bpp contains an optimisation to avoid reading the screen pixel if the text pixel is fully opaque. However this means that the source alpha can't be preserved, going against the rules set out in the comment preceeding setblendingdata (and in reality, the alpha channel would be set to zero). Fix this by introducing a new blend_putdata_32bppA variant which always reads from the screen, ensuring the alpha channel can be preserved.
Admin:
  Tested on Raspberry Pi 3B
  Hardware overlays underneath the desktop are no longer visible through the zero-alpha font pixels


Version 3.80. Tagged as 'Manager-3_80'

VersionASM

@@ -11,13 +11,13 @@
                         GBLS    Module_HelpVersion
                         GBLS    Module_ComponentName
                         GBLS    Module_ComponentPath
-Module_MajorVersion     SETS    "3.79"
-Module_Version          SETA    379
+Module_MajorVersion     SETS    "3.80"
+Module_Version          SETA    380
 Module_MinorVersion     SETS    ""
-Module_Date             SETS    "04 Feb 2018"
-Module_ApplicationDate  SETS    "04-Feb-18"
+Module_Date             SETS    "13 Aug 2018"
+Module_ApplicationDate  SETS    "13-Aug-18"
 Module_ComponentName    SETS    "Manager"
 Module_ComponentPath    SETS    "castle/RiscOS/Sources/Video/Render/Fonts/Manager"
-Module_FullVersion      SETS    "3.79"
-Module_HelpVersion      SETS    "3.79 (04 Feb 2018)"
+Module_FullVersion      SETS    "3.80"
+Module_HelpVersion      SETS    "3.80 (13 Aug 2018)"
                         END

VersionNum

-/* (3.79)
+/* (3.80)
  *
  * This file is automatically maintained by srccommit, do not edit manually.
  * Last processed by srccommit version: 1.1.
  *
  */
-#define Module_MajorVersion_CMHG        3.79
+#define Module_MajorVersion_CMHG        3.80
 #define Module_MinorVersion_CMHG        
-#define Module_Date_CMHG                04 Feb 2018
+#define Module_Date_CMHG                13 Aug 2018
 
-#define Module_MajorVersion             "3.79"
-#define Module_Version                  379
+#define Module_MajorVersion             "3.80"
+#define Module_Version                  380
 #define Module_MinorVersion             ""
-#define Module_Date                     "04 Feb 2018"
+#define Module_Date                     "13 Aug 2018"
 
-#define Module_ApplicationDate          "04-Feb-18"
+#define Module_ApplicationDate          "13-Aug-18"
 
 #define Module_ComponentName            "Manager"
 #define Module_ComponentPath            "castle/RiscOS/Sources/Video/Render/Fonts/Manager"
 
-#define Module_FullVersion              "3.79"
-#define Module_HelpVersion              "3.79 (04 Feb 2018)"
-#define Module_LibraryVersionInfo       "3:79"
+#define Module_FullVersion              "3.80"
+#define Module_HelpVersion              "3.80 (13 Aug 2018)"
+#define Module_LibraryVersionInfo       "3:80"

s/Blending

@@ -165,6 +165,12 @@ setblendingdata EntryS "R0-R3,R10"
         LDMIA   R1, {R0, R1}                            ; get blending functions
         CMP     R0, #0
         BEQ     %FT96
+        ; Use the right variant for 32bpp with alpha
+        CMP     R0, #blend_convert_32bpp - blend_table
+        BNE     %FT20
+        TST     R2, #ModeFlag_DataFormatSub_Alpha
+        ADDNE   R0, R0, #blend_putdata_32bppA - blend_putdata_32bpp
+20
         ADD     R0, R0, R3
         ADD     R1, R1, R3
         ADRL    R3, blend_putdata
@@ -539,6 +545,24 @@ blend_putdata_32bpp_common
 
 	Pull	"R1-R3, PC"
 
+; Variant of blend_putdata_32bpp for alpha channel destination - always pixel up destination word, so that alpha channel is preserved
+blend_putdata_32bppA
+
+	ANDS	outdata, outdata, outmask		; ensure only relevant bits are set
+	BEQ	blend_nodata
+
+	Push	"R1-R3, LR"
+
+	LDR	R1, blend_fgvalue			; get the foreground painting colour
+
+        ; outdata = &00000000-&00001000 (clear->solid)
+	MOV	outdata, outdata, LSR # 8		; amount of foreground to blend
+        ; outdata = 0-16 (clear-solid)
+	RSB	LR, outdata, # 16			;   and the amount of background
+
+        LDR     pchar, [ outptr ]			; and pick up the current background pixel data
+        B       blend_putdata_32bpp_common
+
 blend_putdataM_32bpp
 
 	ANDS	outdata, outdata, outmask