sRGB support in Ganesh. Several pieces:

sRGB support now also requires GL_EXT_texture_sRGB_decode, which allows
us to disable sRGB -> Linear conversion when reading textures. This gives
us an easy way to support "legacy" L32 mode. We disable decoding based on
the pixel config of the render target. Textures can override that behavior
(specifically for format-conversion draws where we want that behavior).

Added sBGRA pixel config, which is not-really-a-format. It's just sRGBA
internally, and the external format is BGR order, so TexImage calls will
swizzle correctly. This lets us interact with sRGB raster surfaces on BGR
platforms.

Devices without sRGB support behave like they always have: conversion from
color type and profile type ignores sRGB and always returns linear pixel
configs.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1789663002

Review URL: https://codereview.chromium.org/1789663002
diff --git a/src/gpu/gl/GrGLDefines.h b/src/gpu/gl/GrGLDefines.h
index cd73be4..ff4c457 100644
--- a/src/gpu/gl/GrGLDefines.h
+++ b/src/gpu/gl/GrGLDefines.h
@@ -506,12 +506,17 @@
 /* TextureUsage */
 #define GR_GL_FRAMEBUFFER_ATTACHMENT         0x93A3
 
+/* TextureSRGBDecode */
+#define GR_GL_DECODE_EXT                     0x8A49
+#define GR_GL_SKIP_DECODE_EXT                0x8A4A
+
 /* TextureParameterName */
 #define GR_GL_TEXTURE_MAG_FILTER             0x2800
 #define GR_GL_TEXTURE_MIN_FILTER             0x2801
 #define GR_GL_TEXTURE_WRAP_S                 0x2802
 #define GR_GL_TEXTURE_WRAP_T                 0x2803
 #define GR_GL_TEXTURE_USAGE                  0x93A2
+#define GR_GL_TEXTURE_SRGB_DECODE_EXT        0x8A48
 
 /* TextureTarget */
 /*      GL_TEXTURE_2D */