Add Compat Limits to Proposal (gpuweb#4397)

* Add Compat Limits Proposal

proposals/compatibility-mode.md

@@ -109,6 +109,39 @@ During createRenderPipeline(), GPUDepthStencilState.depthBiasClamp must be zero,
 
 **Justification**: GLSL ES 3.1 does not support glPolygonOffsetClamp().
 
+### 10. Lower limits.
+
+The differences in limits between compatibility mode and standard WebGPU
+are as follows
+
+
+| limit                               | compat  | standard  | gl limit                                     |
+| :---------------------------------- | ------: | --------: | :------------------------------------------- |
+| `maxColorAttachments`               |       4 |         8 | min(MAX_COLOR_ATTACHMENTS, MAX_DRAW_BUFFERS) |
+| `maxComputeInvocationsPerWorkgroup` |     128 |       256 | MAX_COMPUTE_WORK_GROUP_INVOCATIONS           |
+| `maxComputeWorkgroupSizeX`          |     128 |       256 | MAX_COMPUTE_WORK_GROUP_SIZE                  |
+| `maxComputeWorkgroupSizeY`          |     128 |       256 | MAX_COMPUTE_WORK_GROUP_SIZE                  |
+| `maxInterStageShaderVariables`      |      15 |        16 | MAX_VARYING_VECTORS                          |
+| `maxStorageBuffersPerShaderStage`   |       4 |         8 | min(GL_MAX_SHADER_STORAGE_BUFFER_BINDINGS, GL_MAX_VERTEX_SHADER_STORAGE_BLOCKS, GL_MAX_FRAGMENT_SHADER_STORAGE_BLOCKS, GL_MAX_COMPUTE_SHADER_STORAGE_BLOCKS) |
+| `maxTextureDimension1D`             |    4096 |      8192 | MAX_TEXTURE_SIZE                             |
+| `maxTextureDimension2D`             |    4096 |      8192 | MAX_TEXTURE_SIZE                             |
+| `maxUniformBufferBindingSize`       |   16384 |     65536 | MAX_UNIFORM_BLOCK_SIZE                       |
+| `maxVertexAttributes`        | 16<sup>a</sup> |        16 | MAX_VERTEX_ATTRIBS                           |
+
+(a) In compatibility mode, using `@builtin(vertex_index)`
+and/or `@builtin(instance_index)` each count as an
+attribute.
+
+Note: Some of the limits are derived from a survey of OpenGL ES 3.1 devices
+and are higher than the limit specified in the OpenGL ES 3.1 spec.
+
+For example, in OpenGL ES 3.1, GL_MAX_FRAGMENT_IMAGE_UNIFORMS and GL_MAX_VERTEX_IMAGE_UNIFORMS can be
+zero but `maxStorageTexturesPerShaderStage` is 4 above as all 3.1 devices support at
+least 4 of each.
+
+Similar limits include GL_MAX_TEXTURE_SIZE (2048) and GL_MAX_3D_TEXTURE_SIZE (256) but actual
+devices support the values above.
+
 ## Issues
 
 Q: OpenGL ES does not have "coarse" and "fine" variants of the derivative instructions (`dFdx()`, `dFdy()`, `fwidth()`). Should WGSL's "fine" derivatives (`dpdxFine()`, `dpdyFine()`, and `fwidthFine()`) be required to deliver high precision results? See [Issue 4325](https://github.com/gpuweb/gpuweb/issues/4325).