Fix #147618: PyGPU incorrect colors when drawing images

644fb2b679 fixed a long standing issue
that offscreen example showed the wrong colors. However the fix assumes
that input texture color space is always sRGB.

This adds a shader variation that draws textures that are stored in scene referred
linear color space (like all of our Image data-block).

Co-authored-by: Clément Foucault <foucault.clem@gmail.com>
Pull Request: https://projects.blender.org/blender/blender/pulls/147788

doc/python_api/examples/gpu.6.py

@@ -12,7 +12,7 @@ IMAGE_NAME = "Untitled"
 image = bpy.data.images[IMAGE_NAME]
 texture = gpu.texture.from_image(image)
 
-shader = gpu.shader.from_builtin('IMAGE')
+shader = gpu.shader.from_builtin('IMAGE_SCENE_LINEAR_TO_REC709_SRGB')
 batch = batch_for_shader(
     shader, 'TRI_FAN',
     {
@@ -45,7 +45,7 @@ IMAGE_NAME = "Untitled"
 image = bpy.data.images[IMAGE_NAME]
 texture = gpu.texture.from_image(image)
 
-shader = gpu.shader.from_builtin('IMAGE')
+shader = gpu.shader.from_builtin('IMAGE_SCENE_LINEAR_TO_REC709_SRGB')
 batch = batch_for_shader(
     shader, 'TRIS',
     {