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

source/blender/python/gpu/gpu_py_shader.c

@@ -46,6 +46,9 @@
   "``3D_FLAT_COLOR``\n" \
   "   :Attributes: vec3 pos, vec4 color\n" \
   "   :Uniforms: none\n" \
+  "``3D_IMAGE``\n" \
+  "   :Attributes: vec3 pos, vec2 texCoord\n" \
+  "   :Uniforms: sampler2D image\n" \
   "``3D_SMOOTH_COLOR``\n" \
   "   :Attributes: vec3 pos, vec4 color\n" \
   "   :Uniforms: none\n" \
@@ -68,6 +71,7 @@ static const struct PyC_StringEnumItems pygpu_shader_builtin_items[] = {
     {GPU_SHADER_2D_SMOOTH_COLOR, "2D_SMOOTH_COLOR"},
     {GPU_SHADER_2D_UNIFORM_COLOR, "2D_UNIFORM_COLOR"},
     {GPU_SHADER_3D_FLAT_COLOR, "3D_FLAT_COLOR"},
+    {GPU_SHADER_3D_IMAGE, "3D_IMAGE"},
     {GPU_SHADER_3D_SMOOTH_COLOR, "3D_SMOOTH_COLOR"},
     {GPU_SHADER_3D_UNIFORM_COLOR, "3D_UNIFORM_COLOR"},
     {GPU_SHADER_3D_POLYLINE_FLAT_COLOR, "3D_POLYLINE_FLAT_COLOR"},