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

1 files changed, 10 insertions, 0 deletions

diff --git a/source/blender/gpu/GPU_shader.h b/source/blender/gpu/GPU_shader.h
index b620fe9cc9d..72a36ebfac4 100644
--- a/source/blender/gpu/GPU_shader.h
+++ b/source/blender/gpu/GPU_shader.h
@@ -280,6 +280,16 @@ typedef enum eGPUBuiltinShader {
   GPU_SHADER_2D_IMAGE_OVERLAYS_STEREO_MERGE,
   GPU_SHADER_2D_IMAGE_SHUFFLE_COLOR,
   /**
+   * Draw a texture in 3D. Take a 3D position and a 2D texture coordinate for each vertex.
+   *
+   * Exposed via pyapi for add-ons.
+   *
+   * \param image: uniform sampler2D
+   * \param texCoord: in vec2
+   * \param pos: in vec3
+   */
+  GPU_SHADER_3D_IMAGE,
+  /**
    * Draw texture with alpha. Take a 3D position and a 2D texture coordinate for each vertex.
    *
    * \param alpha: uniform float