Allow building targets that do not create metal shaders on all platforms. (flutter#133)

Author: dnfield

impeller/BUILD.gn

@@ -12,6 +12,10 @@ config("impeller_public_config") {
   if (impeller_supports_platform) {
     defines += [ "IMPELLER_SUPPORTS_PLATFORM=1" ]
   }
+
+  if (is_win) {
+    defines += [ "_USE_MATH_DEFINES" ]
+  }
 }
 
 group("impeller") {

impeller/tessellator/BUILD.gn

@@ -15,8 +15,14 @@ impeller_component("tessellator") {
   deps = [ "//third_party/libtess2" ]
 }
 
-shared_library("tessellator_shared") {
-  output_name = "tessellator"
+impeller_component("tessellator_shared") {
+  target_type = "shared_library"
+  if (is_win) {
+    output_name = "libtessellator"
+  } else {
+    output_name = "tessellator"
+  }
+
 
   sources = [
     "c/tessellator.h",

impeller/tools/impeller.gni

@@ -10,7 +10,10 @@ declare_args() {
   impeller_enable_playground = false
 
   # Whether Impeller is supported on the platform.
-  impeller_supports_platform = is_mac || is_ios
+  impeller_supports_platform = true
+
+  # Whether Impeller shaders are supported on the platform.
+  impeller_shaders_supports_platform = is_mac || is_ios
 }
 
 # ------------------------------------------------------------------------------
@@ -289,7 +292,7 @@ template("impeller_shaders_real") {
 # @see             impeller_shaders_real
 #
 template("impeller_shaders") {
-  if (impeller_supports_platform) {
+  if (impeller_shaders_supports_platform) {
     impeller_shaders_real(target_name) {
       forward_variables_from(invoker, "*")
     }