[core] split zeroing and dd write operations

* This is in preparation for async reads
* Also move open/close image operations to WriteDrive()
* Also increase DD buffer size to 32 MB to improve performance

src/rufus.h

@@ -99,12 +99,12 @@
 #define BADCLOCK_PATTERN_MLC        {0x00, 0xff, 0x33, 0xcc}
 #define BADBLOCK_PATTERN_TLC        {0x00, 0xff, 0x1c71c7, 0xe38e38}
 #define BADBLOCK_BLOCK_SIZE         (128 * 1024)
-#define LARGE_FAT32_SIZE            (32*1073741824LL)	// Size at which we need to use fat32format
+#define LARGE_FAT32_SIZE            (32 * 1073741824LL)	// Size at which we need to use fat32format
 #define UDF_FORMAT_SPEED            3.1f		// Speed estimate at which we expect UDF drives to be formatted (GB/s)
 #define UDF_FORMAT_WARN             20			// Duration (in seconds) above which we warn about long UDF formatting times
 #define MAX_FAT32_SIZE              2.0f		// Threshold above which we disable FAT32 formatting (in TB)
 #define FAT32_CLUSTER_THRESHOLD     1.011f		// For FAT32, cluster size changes don't occur at power of 2 boundaries but sligthly above
-#define DD_BUFFER_SIZE              65536		// Minimum size of the buffer we use for DD operations
+#define DD_BUFFER_SIZE              (32 * 1024 * 1024)	// Minimum size of buffer to use for DD operations
 #define UBUFFER_SIZE                4096
 #define RSA_SIGNATURE_SIZE          256
 #define CBN_SELCHANGE_INTERNAL      (CBN_SELCHANGE + 256)