Fixed bug 1943 - Wrong handling of legacy 32bpp BMP files

Kang Seonghoon

While BMP format supports alpha channel, it is enabled only when the header is at least 56 bytes long (BITMAPV3INFOHEADER and later). For very common 40-byte-long header (BITMAPINFOHEADER) 32bpp format should be interpreted as BGRX format, but currently SDL interprets them as BGRA format and causes a significant compatibility problem as many 32bpp files use a padding byte of 0 ("transparent" in BGRA interpretation).

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I fixed this by checking to see if the alpha channel is all 0, and if so, setting it opaque.