EEVEE & Viewport: Add a built-in shader called 3D_IMAGE, and expose to the python API

Adds an example python script to the documentation for the 3D_IMAGE shader.

The **use-case** is to draw textures with 3D vertex positions, in XR views as well as non-XR views (in a simpler manner).

**Testing**: I've tested that this compiles and works on my Macbook (with the example python script included in this change). I don't have access to a Windows or Linux machine right now, but this change doesn't look platform-specific and no new glsl shaders have been added or edited by this change. I'll try to get access to a Windows machine, but if someone does have one, I'd be really grateful if they could try this change. Thanks!

**Problem addressed**: The existing 2D_IMAGE shader (exposed in the python API) gets near-clipped when drawn in the
XR view, regardless of the near-clip settings. Additionally, the 2D_IMAGE shader only accepts 2D
positions for the image vertices, which means drawing textures in 3D requires providing
2D coordinates and then pushing a transform-rotate-scale matrix to the GPU, even for
non-XR (i.e. WINDOW) views. The 3D_IMAGE shader is simpler: it accepts 3D vertex positions, and doesn't require
any additional work by the scripter.

**Workaround**: The current workaround is to use custom shaders in the python script.

**Non-intrusive change**: No new glsl shaders were added. This change just bundles two existing shaders: the vertex shader used
by the 3D_IMAGE_MODULATE_ALPHA shader, and the fragment shader used by the 2D_IMAGE shader.

Reviewed By: #eevee_viewport, jbakker

Differential Revision: https://developer.blender.org/D14832

doc/python_api/examples/gpu.6.py

@@ -29,3 +29,36 @@ def draw():
 
 
 bpy.types.SpaceView3D.draw_handler_add(draw, (), 'WINDOW', 'POST_PIXEL')
+
+"""
+3D Image
+--------
+
+Similar to the 2D Image shader, but works with 3D positions for the image vertices.
+To use this example you have to provide an image that should be displayed.
+"""
+import bpy
+import gpu
+from gpu_extras.batch import batch_for_shader
+
+IMAGE_NAME = "Untitled"
+image = bpy.data.images[IMAGE_NAME]
+texture = gpu.texture.from_image(image)
+
+shader = gpu.shader.from_builtin('3D_IMAGE')
+batch = batch_for_shader(
+    shader, 'TRIS',
+    {
+        "pos": ((0, 0, 0), (0, 1, 1), (1, 1, 1), (1, 1, 1), (1, 0, 0), (0, 0, 0)),
+        "texCoord": ((0, 0), (0, 1), (1, 1), (1, 1), (1, 0), (0, 0)),
+    },
+)
+
+
+def draw():
+    shader.bind()
+    shader.uniform_sampler("image", texture)
+    batch.draw(shader)
+
+
+bpy.types.SpaceView3D.draw_handler_add(draw, (), 'WINDOW', 'POST_VIEW')