VaapiDrmPicture: use RGBX format instead of RGBA
VaapiDrmPicture: use RGBX format instead of RGBA

The format here directly influences the GL texture format. See
GLImageOzoneNativePixmap::Initialize() where FourCC(format) is called.

In general the alpha for video is undefined. On gen7 intel libva driver
actually takes care of filling alpha to 255, but this is not feasible
for gen5-6.

Note that based on my code inspection, we are really treating the enum
"gfx::BufferFormat::RGBX_8888" as a BGRX format, so I'll propose to
rename it to avoid confusion in a separate CL.

BUG=521826
TEST=verify on lumpy_freon that the bug is fixed.
TBR=posciak

Review URL: https://codereview.chromium.org/1304613003

Cr-Commit-Position: refs/heads/master@{#344546}
(cherry picked from commit abab7eb0cecd4d0ca6f6a7a001eb21c8ebb0c1a8)

Review URL: https://codereview.chromium.org/1309453002 .

Cr-Commit-Position: 62bea078bfe806fd4d8dc2d3869914b6c8093498@{#386}
Cr-Branched-From: ef8f355264e75087b06072b03d328c50bf72843a@{#338390}

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