gpu: Add the GSK_GPU_IMAGE_SRGB flag

This commit just adds the flag, but I wanted to make it an individual
commit to explain the purpose:

The SRGB flag is meant to be used for images that have an SRGB format.
In Vulkan terms, that means VK_FORMAT_*_SRGB.
In GL, it means GL_SRGB or GL_SRGB_ALPHA.

As these formats have been madatory since GL 3.0, we can (ab)use them
uncoditionally. Images in these formats are renderable, too, so it's
not just usable for uploading.

What these images allow is treating the data as sRGB while shaders
access them as linear, thereby getting sRGB<=>linear conversions for
free.

It is also possible to switch off the linearization of these images and
treat them as sRGB, which allows all sorts of shenanigans, though one
has to be careful if that turning off applies to the relevant GL/Vulkan
code in question.

gsk/gpu/gskgputypesprivate.h

@@ -30,6 +30,7 @@ typedef enum {
   GSK_GPU_IMAGE_MIPMAP         = (1 << 5),
   GSK_GPU_IMAGE_FILTERABLE     = (1 << 6),
   GSK_GPU_IMAGE_RENDERABLE     = (1 << 7),
+  GSK_GPU_IMAGE_SRGB           = (1 << 8),
 } GskGpuImageFlags;
 
 typedef enum {