ui: Rename gfx::BufferUsage enum values and add BufferUsage::GPU_READ.

The problem with the current usage modes is that we currently use
SCANOUT for anything that doesn't require CPU access. When importing
a buffer we typically don't care about anything but having the GPU
be able to read from it. SCANOUT is not a good description of that
and currently means GPU read/write support.

This patch attempts to make usage more explicit and it solves the
use case where a buffer type can only provide GPU read and not GPU
write support (e.g. SHM fallback). Here are the new usage modes:

- GPU_READ
- GPU_READ_WRITE
- GPU_READ_CPU_READ_WRITE

GPU_READ is useful when importing a buffer and all you care about is
being able to use it for sampling and optionally scanout.

GPU_READ_WRITE is useful for things like WebGL or GPU raster where
we want the ability to write to the buffer using GL and potentially
use it for scanout.

GPU_READ_CPU_READ_WRITE is the usage mode that allows CPU access and
is used for one-copy and zero-copy texture initialization. There's
nothing preventing this mode from also supporting scanout.

- GPU_READ_CPU_READ_WRITE_PERSISTENT

This usage mode indicates that the data will be persistent across
Unmap()/Map() calls but will likely become part of the normal
GPU_READ_CPU_READ_WRITE usage once all buffer types support this.

BUG=538325
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1420923005

Cr-Commit-Position: refs/heads/master@{#356108}

1 files changed, 1 insertions, 2 deletions

diff --git a/ui/gl/gl_bindings.h b/ui/gl/gl_bindings.h
index c1ad779..f043bef 100644
--- a/ui/gl/gl_bindings.h
+++ b/ui/gl/gl_bindings.h
@@ -127,8 +127,7 @@
 #define GL_COMMANDS_COMPLETED_CHROMIUM                   0x84F7
 
 // GL_CHROMIUM_gpu_memory_buffer_image
-#define GL_MAP_CHROMIUM                                  0x78F1
-#define GL_SCANOUT_CHROMIUM                              0x78F2
+#define GL_READ_WRITE_CHROMIUM                           0x78F2
 
 // GL_CHROMIUM_yuv_420_image
 #define GL_RGB_YUV_420_CHROMIUM                          0x78FA